feat(lotw): cancel in-flight sync, honest progress bar, drop stale hint

- Cancel: the dialog's cancel/back while syncing now aborts the download job
  (cooperative cancellation in the stream reader, CancellationException
  propagated, nothing written to storage). An accidental full sync can be
  stopped immediately.
- Progress: the size estimate is conservative (900 B/record vs measured 720)
  so the bar never hits 100% before the download truly ends; storage writes
  moved to the IO dispatcher (multi-MB per-grid QSO JSON used to freeze the
  main thread after download); when the bar caps, the UI switches to
  'Processing downloaded data…' instead of looking stuck.
- Removed the outdated '3-10 seconds' hint (misleading for large accounts).

core/data/src/main/java/com/rtbishop/look4sat/core/data/repository/LoTWRepository.kt

@@ -22,7 +22,9 @@ import com.rtbishop.look4sat.core.domain.repository.LoTWPhase
 import com.rtbishop.look4sat.core.domain.repository.LoTWProgress
 import com.rtbishop.look4sat.core.domain.repository.LoTWResult
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ensureActive
 import kotlinx.coroutines.withContext
+import kotlin.coroutines.coroutineContext
 import java.net.HttpURLConnection
 import java.net.SocketTimeoutException
 import java.net.URL
@@ -91,7 +93,7 @@ class LoTWRepository : ILoTWRepository {
         return Triple(grids, qsos, roamed)
     }
 
-    private fun fetchReportBody(
+    private suspend fun fetchReportBody(
         callsign: String,
         password: String,
         since: String,
@@ -151,6 +153,9 @@ class LoTWRepository : ILoTWRepository {
                 !body.contains("<eoh>", ignoreCase = true) -> Result.failure(RateLimitException())
                 else -> Result.success(body)
             }
+        } catch (e: java.util.concurrent.CancellationException) {
+            // Cancellation must propagate — it is not a network failure.
+            throw e
         } catch (e: SocketTimeoutException) {
             Result.failure(TimeoutException(e.message ?: "timed out"))
         } catch (e: SSLException) {
@@ -167,7 +172,7 @@ class LoTWRepository : ILoTWRepository {
      * body size is estimated as NUMREC * AVG_RECORD_BYTES so the UI can show a
      * meaningful progress bar and remaining-time estimate.
      */
-    private fun readBodyWithProgress(
+    private suspend fun readBodyWithProgress(
         stream: java.io.InputStream,
         isGzip: Boolean,
         onProgress: (LoTWProgress) -> Unit
@@ -185,6 +190,11 @@ class LoTWRepository : ILoTWRepository {
         var lastSampleBytes = 0L
         var lastEmitMs = 0L
         while (true) {
+            // Cooperative cancellation: lets the user abort a full sync that
+            // was started by mistake. Throws CancellationException once the
+            // job is cancelled; the blocking read below is fast while chunks
+            // keep arriving, so the abort lands on the next chunk boundary.
+            coroutineContext.ensureActive()
             val n = reader.read(buf)
             if (n <= 0) break
             sb.append(buf, 0, n)
@@ -440,9 +450,11 @@ class LoTWRepository : ILoTWRepository {
         val NUMREC_REGEX = Regex("<APP_LoTW_NUMREC:\\d+>(\\d+)")
         /**
          * Measured 2026-09-16 (BH6RJD, 2367 QSLs): 1.7 MB body streamed at
-         * ~9.5 KB/s ≈ 180 s, i.e. ≈ 720 bytes per ADIF record. Used to predict
-         * the total body size from the header record count.
+         * ~9.5 KB/s ≈ 180 s, i.e. ≈ 720 bytes per ADIF record. Use a
+         * CONSERVATIVE (larger) estimate so the progress bar never hits 100%
+         * before the download truly ends — an under-estimate made the bar sit
+         * at 100% while data was still flowing, read as "finished but stuck".
          */
-        const val AVG_RECORD_BYTES = 720L
+        const val AVG_RECORD_BYTES = 900L
     }
 }

core/presentation/src/main/res/values-zh/strings.xml

@@ -160,7 +160,6 @@
     <string name="prefs_lotw_callsign">LoTW 呼号</string>
     <string name="prefs_lotw_password">LoTW 密码</string>
     <string name="prefs_lotw_hint">直接从 ARRL LoTW 拉取该呼号已确认 QSO 的网格并覆盖本地已存集合（当前 %1$d 个）。密码仅保存在手机本地。点"同步"会自动保存凭据并拉取。</string>
-    <string name="prefs_lotw_sync_duration_hint">同步通常约需 3–10 秒；首次同步或 ARRL 服务器繁忙时可能更久。</string>
     <string name="prefs_lotw_sync">同步</string>
     <string name="prefs_lotw_syncing">同步中…</string>
     <string name="prefs_lotw_configured">已从 LoTW 同步 — 共 %1$d 个网格</string>
@@ -175,6 +174,7 @@
     <string name="lotw_sync_rate_hint">下载速度约每 100 条 QSO 需 8 秒（另有 5–40 秒服务器生成时间）</string>
     <string name="lotw_sync_progress_connecting">正在连接 LoTW 服务器…</string>
     <string name="lotw_sync_progress_downloading">正在下载报告…</string>
+    <string name="lotw_sync_progress_saving">正在处理下载的数据…</string>
     <string name="lotw_sync_progress_count">本次同步 %1$d 条 QSO…</string>
     <string name="lotw_sync_progress_qso">本次同步 %1$d 条 QSO，预计还需 %2$d 秒</string>
     <string name="prefs_locator_text">使用梅登黑德网格设置站点位置</string>

core/presentation/src/main/res/values/strings.xml

@@ -195,7 +195,6 @@
     <string name="prefs_lotw_callsign">LoTW callsign</string>
     <string name="prefs_lotw_password">LoTW password</string>
     <string name="prefs_lotw_hint">Fetch confirmed-QSO grids directly from ARRL LoTW for this callsign and replace the stored set (currently %1$d). The password is stored on this device only. Tapping Sync saves the credentials and fetches in one step.</string>
-    <string name="prefs_lotw_sync_duration_hint">A sync usually takes about 3–10 seconds; the first one or a busy ARRL server can take longer.</string>
     <string name="prefs_lotw_sync">Sync</string>
     <string name="prefs_lotw_syncing">Syncing…</string>
     <string name="prefs_lotw_configured">Synced from LoTW — %1$d grids total</string>
@@ -210,6 +209,7 @@
     <string name="lotw_sync_rate_hint">Download runs at ~8 s per 100 QSOs (plus 5–40 s server generation time)</string>
     <string name="lotw_sync_progress_connecting">Connecting to LoTW…</string>
     <string name="lotw_sync_progress_downloading">Downloading report…</string>
+    <string name="lotw_sync_progress_saving">Processing downloaded data…</string>
     <string name="lotw_sync_progress_count">Syncing %1$d QSOs…</string>
     <string name="lotw_sync_progress_qso">Syncing %1$d QSOs, ~%2$d s remaining</string>
     <string name="prefs_locator_text">Set station\'s position using locator</string>

feature/settings/src/main/java/com/rtbishop/look4sat/feature/settings/SettingsDialog.kt

@@ -233,6 +233,8 @@ fun LoTWDialog(
     progress: com.rtbishop.look4sat.core.domain.repository.LoTWProgress?,
     message: String?,
     dismiss: () -> Unit,
+    /** Called on cancel/back while a sync is running: aborts the job. */
+    onCancelSync: () -> Unit,
     onSave: (com.rtbishop.look4sat.core.domain.model.LoTWSettings) -> Unit,
     onSyncFull: (com.rtbishop.look4sat.core.domain.model.LoTWSettings) -> Unit,
     onSyncIncremental: (com.rtbishop.look4sat.core.domain.model.LoTWSettings) -> Unit
@@ -241,8 +243,10 @@ fun LoTWDialog(
     val pass = rememberSaveable { mutableStateOf(initialSettings.password) }
     SharedDialog(
         title = stringResource(R.string.prefs_lotw_title),
-        onDismissRequest = dismiss,
-        onCancel = dismiss,
+        // While syncing, cancel/back aborts the download instead of merely
+        // hiding the dialog — a full sync started by mistake must be stoppable.
+        onDismissRequest = { if (isSyncing) onCancelSync() else dismiss() },
+        onCancel = { if (isSyncing) onCancelSync() else dismiss() },
         // The two sync buttons below are the primary actions: saving
         // credentials and fetching confirmed grids happen in one step.
         onAccept = null
@@ -276,6 +280,11 @@ fun LoTWDialog(
                 progress == null ||
                     progress.phase == com.rtbishop.look4sat.core.domain.repository.LoTWPhase.Connecting ->
                     stringResource(R.string.lotw_sync_progress_connecting)
+                // Download finished (or the size estimate reached its cap):
+                // parsing and saving may still run — say so instead of the bar
+                // sitting at 100% with no feedback, which reads as "stuck".
+                progress.fraction >= 1f ->
+                    stringResource(R.string.lotw_sync_progress_saving)
                 progress.qsoCount > 0 && progress.remainingSeconds > 0 ->
                     stringResource(R.string.lotw_sync_progress_qso, progress.qsoCount, progress.remainingSeconds)
                 progress.qsoCount > 0 ->
@@ -307,12 +316,6 @@ fun LoTWDialog(
                 color = MaterialTheme.colorScheme.onSurfaceVariant,
                 modifier = Modifier.padding(horizontal = LocalSpacing.current.large)
             )
-            Text(
-                text = stringResource(R.string.prefs_lotw_sync_duration_hint),
-                style = MaterialTheme.typography.bodySmall,
-                color = MaterialTheme.colorScheme.onSurfaceVariant,
-                modifier = Modifier.padding(horizontal = LocalSpacing.current.large)
-            )
         }
         if (message != null) {
             Text(

feature/settings/src/main/java/com/rtbishop/look4sat/feature/settings/SettingsScreen.kt

@@ -223,6 +223,7 @@ private fun SettingsScreen(uiState: SettingsState, onAction: (SettingsAction) ->
             progress = uiState.lotwProgress,
             message = lotwErrorMessage,
             dismiss = { dialogs.lotw = false },
+            onCancelSync = { onAction(SettingsAction.CancelLoTWSync); dialogs.lotw = false },
             onSave = { onAction(SettingsAction.UpdateLoTW(it)) },
             onSyncFull = { onAction(SettingsAction.SyncLoTWGrids(it, LoTWSyncMode.Full)) },
             onSyncIncremental = { onAction(SettingsAction.SyncLoTWGrids(it, LoTWSyncMode.Incremental)) }

feature/settings/src/main/java/com/rtbishop/look4sat/feature/settings/SettingsState.kt

@@ -117,6 +117,8 @@ sealed interface SettingsAction {
         val settings: com.rtbishop.look4sat.core.domain.model.LoTWSettings,
         val mode: LoTWSyncMode
     ) : SettingsAction
+    /** Abort an in-flight LoTW sync (wrong button / changed mind). */
+    data object CancelLoTWSync : SettingsAction
 
     // Update checker
     data object CheckForUpdate : SettingsAction

feature/settings/src/main/java/com/rtbishop/look4sat/feature/settings/SettingsViewModel.kt

@@ -35,6 +35,7 @@ import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
 import java.io.File
 
 class SettingsViewModel(
@@ -49,6 +50,8 @@ class SettingsViewModel(
 
     private val defaultPosSettings = PositionSettings(false, settingsRepo.stationPosition.value, 0)
     private val defaultDataSettings = DataSettings(false, 0, 0, 0L)
+    /** In-flight LoTW sync job, cancelled by CancelLoTWSync. */
+    private var lotwSyncJob: kotlinx.coroutines.Job? = null
     private val _uiState = MutableStateFlow(
         SettingsState(
             appVersionName = settingsRepo.appVersionName,
@@ -159,6 +162,7 @@ class SettingsViewModel(
             // LoTW confirmed grids
             is SettingsAction.UpdateLoTW -> settingsRepo.updateLoTWSettings(action.settings)
             is SettingsAction.SyncLoTWGrids -> syncLoTWGrids(action.settings, action.mode)
+            SettingsAction.CancelLoTWSync -> cancelLoTWSync()
             // Update checker
             SettingsAction.CheckForUpdate -> checkForUpdate()
             SettingsAction.DownloadUpdate -> downloadUpdate()
@@ -215,7 +219,7 @@ class SettingsViewModel(
         _uiState.update {
             it.copy(lotwSyncing = true, lotwSyncMode = effectiveMode, lotwProgress = null, lotwError = null)
         }
-        viewModelScope.launch {
+        lotwSyncJob = viewModelScope.launch {
             val result = lotwRepo.fetchConfirmedGridQsos(callsign, settings.password, since) { progress ->
                 _uiState.update { it.copy(lotwProgress = progress) }
             }
@@ -235,9 +239,6 @@ class SettingsViewModel(
                     val mergedRoamed = if (effectiveMode == LoTWSyncMode.Incremental) {
                         existingRoamed + result.roamedGrids
                     } else result.roamedGrids
-                    settingsRepo.setWorkedGrids(mergedGrids)
-                    settingsRepo.setWorkedGridQsos(mergedQsos)
-                    settingsRepo.setRoamedGrids(mergedRoamed)
                     // Remember when/what we synced, so the next incremental pull
                     // asks LoTW for only the confirmations since today.
                     val today = java.text.SimpleDateFormat("yyyyMMdd", java.util.Locale.US)
@@ -245,6 +246,16 @@ class SettingsViewModel(
                         .format(java.util.Date())
                     settingsRepo.setLastLotwSyncDate(today)
                     settingsRepo.setLastLotwSyncCallsign(callsign)
+                    // Persist on the IO dispatcher: building the per-grid QSO JSON
+                    // and writing SharedPreferences synchronously blocks the
+                    // calling thread, and for large accounts (tens of thousands
+                    // of QSOs → multi-MB JSON) that froze the main thread after
+                    // the download finished — 'bar done but app stuck'.
+                    withContext(kotlinx.coroutines.Dispatchers.IO) {
+                        settingsRepo.setWorkedGrids(mergedGrids)
+                        settingsRepo.setWorkedGridQsos(mergedQsos)
+                        settingsRepo.setRoamedGrids(mergedRoamed)
+                    }
                     _uiState.update { state ->
                         state.copy(
                             lotwSyncing = false, lotwSyncMode = null, lotwProgress = null,
@@ -284,6 +295,13 @@ class SettingsViewModel(
         }
     }
 
+    /** Abort the in-flight LoTW sync job and reset its progress state. */
+    private fun cancelLoTWSync() {
+        lotwSyncJob?.cancel()
+        lotwSyncJob = null
+        _uiState.update { it.copy(lotwSyncing = false, lotwSyncMode = null, lotwProgress = null) }
+    }
+
     /** Merge fresh QSO detail into existing per-grid lists, dedup by call + QSO time. */
     private fun mergeGridQsos(
         existing: Map<String, List<com.rtbishop.look4sat.core.domain.model.GridQso>>,