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- Removed the outdated '3-10 seconds' hint (misleading for large accounts).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/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>