fix(radar): 计算器 offset 改为跟随驱动计算器的转发器读写

per-satellite offset 原先只在 SelectTransmitter 里载入、并按 selectedUuid
的 catnum 保存：重开雷达页读不回（看起来"没保存"），选中被 toggle 掉后
catnum 为 null 静默丢写；且计算器页 chip 仅在 dedupe 后多于一条时渲染，
单转发器卫星（JO-97/RS-44/FO-29/AO-73/QO-100/TO-108/FO-82/QB50P1/
UKUBE-1）根本没有 chip 可点，永远触发不了载入——上游 rt-bishop 在 PR #237
评审时预言过这个加载问题。

改为 offset 跟随"驱动计算器的转发器"：该转发器变化（打开/换星）即载入
已存值，按其 catnum 保存，与收发机选中状态解耦；同时删掉 loadPassData 里
的 workaround 载入与 SelectTransmitter 里的 offset 读写，单一来源、与上游
PR #265 同形便于后续合并。存储格式（satelliteOffsets 单 JSON）不变，
已存的值补丁后直接生效。

验证：:feature:radar:compileDebugKotlin 与 :feature:radar:test 通过；
:core:domain:test up-to-date（domain 未改动）。

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/RadarState.kt

@@ -132,5 +132,12 @@ sealed interface RadarAction {
 
     // Calculator actions
     data class ChangeCalculatorOffset(val offsetKHz: String) : RadarAction
+
+    /**
+     * The transponder currently driving the calculator (null when the satellite has
+     * no linear transponder). The per-satellite offset follows it instead of the
+     * transceiver selection, so it loads and saves even when nothing is selected.
+     */
+    data class CalculatorTransponderChanged(val catnum: Int?) : RadarAction
     data class UpdateCalculatorFrequency(val txHz: Long?, val rxHz: Long?) : RadarAction
 }

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/RadarViewModel.kt

@@ -69,6 +69,7 @@ class RadarViewModel(
     private val stationPos = settingsRepo.stationPosition.value
     private val magDeclination = sensorsRepo.getMagDeclination(stationPos)
     private var transponders: List<SatRadio> = emptyList()
+    private var calculatorCatnum: Int? = null
     private var sstvDecoder: SstvDecoder? = null
     private var sstvRecordingJob: Job? = null
     private var cwDecoder: CwDecoder? = null
@@ -158,12 +159,8 @@ class RadarViewModel(
         transponders = allRadios.filter { it.downlinkLow != null }
         if (allRadios.isNotEmpty()) {
             val firstUuid = allRadios.first().uuid
-            val offsetKHz = allRadios.first().catnum?.let { settingsRepo.getSatelliteOffset(it) } ?: ""
             _uiState.update {
-                it.copy(
-                    transceivers = it.transceivers.copy(selectedUuid = firstUuid),
-                    calculatorOffsetKHz = offsetKHz
-                )
+                it.copy(transceivers = it.transceivers.copy(selectedUuid = firstUuid))
             }
             transponders.find { it.uuid == firstUuid }?.let { trackingService.setTransponder(it) }
         }
@@ -241,15 +238,8 @@ class RadarViewModel(
                 // Compute toggle state before the update so we don't read post-update value
                 val isTogglingOff = _uiState.value.transceivers.selectedUuid == action.uuid
                 val newUuid = if (isTogglingOff) null else action.uuid
-                val offsetKHz = if (!isTogglingOff) {
-                    transponders.find { it.uuid == action.uuid }
-                        ?.catnum?.let { settingsRepo.getSatelliteOffset(it) } ?: ""
-                } else ""
                 _uiState.update {
-                    it.copy(
-                        transceivers = it.transceivers.copy(selectedUuid = newUuid),
-                        calculatorOffsetKHz = offsetKHz
-                    )
+                    it.copy(transceivers = it.transceivers.copy(selectedUuid = newUuid))
                 }
                 // Only update the tracking service when selecting a different transponder to
                 // avoid resetting a user-adjusted TX base on re-expand
@@ -319,7 +309,9 @@ class RadarViewModel(
                 _uiState.update { it.copy(cw = it.cw.copy(isExpanded = action.expanded)) }
             }
             is RadarAction.ChangeCalculatorOffset -> {
-                val catnum = _uiState.value.transceivers.selectedUuid?.let { uuid ->
+                // Keyed by the transponder driving the calculator, so the offset saves
+                // even when no transceiver is selected (or selection was toggled off)
+                val catnum = calculatorCatnum ?: _uiState.value.transceivers.selectedUuid?.let { uuid ->
                     transponders.find { it.uuid == uuid }?.catnum
                 }
                 if (catnum != null) {
@@ -327,6 +319,11 @@ class RadarViewModel(
                 }
                 _uiState.update { it.copy(calculatorOffsetKHz = action.offsetKHz) }
             }
+            is RadarAction.CalculatorTransponderChanged -> {
+                calculatorCatnum = action.catnum
+                val offsetKHz = action.catnum?.let { settingsRepo.getSatelliteOffset(it) } ?: ""
+                _uiState.update { it.copy(calculatorOffsetKHz = offsetKHz) }
+            }
             is RadarAction.UpdateCalculatorFrequency -> {
                 _uiState.update {
                     if (it.calculatorTxHz == action.txHz && it.calculatorRxHz == action.rxHz) it

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/TransceiversPage.kt

@@ -176,6 +176,13 @@ fun CalculatorPage(
             ?: calculatorTransceivers.firstOrNull()
     }
 
+    // Keep the per-satellite offset in sync with the transponder driving the
+    // calculator: load the saved value on open/satellite change, and remember
+    // its catnum so editing saves even when no transceiver is selected
+    LaunchedEffect(selectedTransceiver?.uuid) {
+        onAction(RadarAction.CalculatorTransponderChanged(selectedTransceiver?.catnum))
+    }
+
     if (selectedTransceiver == null) {
         EmptyTransceiversContent(modifier)
         return