feat: remember per-satellite doppler offset in calculator

Store the last entered offset (kHz) per satellite in SharedPreferences
via SettingsRepo, and restore it when the linear transponder calculator
is shown for that satellite.

Follows the project's MVI/clean architecture:
- ISettingsRepo: getSatelliteOffset / setSatelliteOffset
- SettingsRepo: backed by SharedPreferences
- RadarState: calculatorOffsetKHz field
- RadarAction: ChangeCalculatorOffset
- RadarViewModel: loads offset on SelectTransmitter, saves on change
- CalculatorPage/DopplerFrequencyCalculator: receive offset via params
instead of accessing SharedPreferences directly from the UI layer.

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

@@ -198,7 +198,8 @@ private fun PagerCard(
                         transceivers = uiState.transceivers.transmitters,
                         selectedUuid = uiState.transceivers.selectedUuid,
                         orbitalPos = uiState.orbitalPos,
-                        onAction = onAction
+                        onAction = onAction,
+                        calculatorOffsetKHz = uiState.calculatorOffsetKHz
                     )
                     RadarPage.Sstv -> SstvPage(
                         sstv = uiState.sstv,

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

@@ -63,7 +63,8 @@ data class RadarState(
     val moonPosition: CelestialComputer.MoonPosition? = null,
     val transceivers: TransceiverSubState = TransceiverSubState(),
     val radioControl: RadioControlSubState = RadioControlSubState(),
-    val sstv: SstvSubState = SstvSubState()
+    val sstv: SstvSubState = SstvSubState(),
+    val calculatorOffsetKHz: String = ""
 )
 
 enum class SstvStatus { Idle, Recording }
@@ -97,4 +98,7 @@ sealed interface RadarAction {
     data object SstvReset : RadarAction
     data class SstvSelectMode(val modeName: String) : RadarAction
     data class SstvPermissionResult(val granted: Boolean) : RadarAction
+
+    // Calculator actions
+    data class ChangeCalculatorOffset(val offsetKHz: String) : RadarAction
 }

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

@@ -260,7 +260,17 @@ 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
-                _uiState.update { it.copy(transceivers = it.transceivers.copy(selectedUuid = newUuid)) }
+                // Load the saved offset for the newly selected satellite
+                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
+                    )
+                }
                 // Only update the tracking service when selecting a different transponder to
                 // avoid resetting a user-adjusted TX base on re-expand
                 if (!isTogglingOff) {
@@ -309,6 +319,15 @@ class RadarViewModel(
                 sstvDecoder?.clearPixels()
                 _uiState.update { it.copy(sstv = it.sstv.copy(currentFrame = null)) }
             }
+            is RadarAction.ChangeCalculatorOffset -> {
+                val catnum = _uiState.value.transceivers.selectedUuid?.let { uuid ->
+                    transponders.find { it.uuid == uuid }?.catnum
+                }
+                if (catnum != null) {
+                    settingsRepo.setSatelliteOffset(catnum, action.offsetKHz)
+                }
+                _uiState.update { it.copy(calculatorOffsetKHz = action.offsetKHz) }
+            }
         }
     }
 

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

@@ -128,6 +128,7 @@ fun CalculatorPage(
     selectedUuid: String?,
     orbitalPos: OrbitalPos?,
     onAction: (RadarAction) -> Unit,
+    calculatorOffsetKHz: String = "",
     modifier: Modifier = Modifier
 ) {
     val calculatorTransceivers = remember(transceivers) {
@@ -180,6 +181,8 @@ fun CalculatorPage(
             DopplerFrequencyCalculator(
                 transponder = selectedTransceiver,
                 orbitalPos = orbitalPos,
+                offsetKHz = calculatorOffsetKHz,
+                onOffsetChange = { onAction(RadarAction.ChangeCalculatorOffset(it)) },
                 modifier = Modifier.fillMaxWidth()
             )
         }
@@ -521,6 +524,8 @@ private enum class EditedField { TX, PASSBAND, RX }
 private fun DopplerFrequencyCalculator(
     transponder: SatRadio,
     orbitalPos: OrbitalPos?,
+    offsetKHz: String = "",
+    onOffsetChange: (String) -> Unit = {},
     modifier: Modifier = Modifier
 ) {
     if (orbitalPos == null || !DopplerFrequencyCalculator.isLinearTransponder(transponder)) return
@@ -529,7 +534,6 @@ private fun DopplerFrequencyCalculator(
     var txFrequencyHz by remember { mutableStateOf(0L) }
     var rxFrequencyHz by remember { mutableStateOf(0L) }
     var passbandPosition by remember { mutableStateOf(0.5f) }
-    var offsetKHz by remember { mutableStateOf("") }
     var stepSizeKHz by remember { mutableIntStateOf(1) }
 
     val offsetHz = offsetKHz.toDoubleOrNull()?.let { it * 1000 }?.toLong() ?: 0L
@@ -712,7 +716,7 @@ private fun DopplerFrequencyCalculator(
                 ) {
                     BasicTextField(
                         value = offsetKHz,
-                        onValueChange = { offsetKHz = it },
+                        onValueChange = onOffsetChange,
                         singleLine = true,
                         textStyle = TextStyle(
                             fontSize = 16.sp,