fix: keep calculator page content composed while scrolling

Store the offset (kHz) per NORAD catalog id in SharedPreferences so
each satellite's last entered offset is restored when reopening the
frequency calculator.

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

@@ -18,6 +18,7 @@
 package com.rtbishop.look4sat.feature.radar
 
 import android.app.Activity
+import android.content.Context
 import android.view.LayoutInflater
 import androidx.compose.animation.AnimatedVisibility
 import androidx.compose.animation.expandVertically
@@ -26,6 +27,8 @@ import androidx.compose.foundation.background
 import androidx.compose.foundation.border
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.interaction.MutableInteractionSource
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.text.BasicTextField
@@ -66,6 +69,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -160,59 +164,53 @@ fun CalculatorPage(
         return
     }
 
-    LazyColumn(
-        modifier = modifier.fillMaxSize(),
+    Column(
+        modifier = modifier
+            .fillMaxSize()
+            .verticalScroll(rememberScrollState()),
         verticalArrangement = Arrangement.spacedBy(10.dp)
     ) {
         if (calculatorTransceivers.size > 1) {
-            item {
-                FlowRow(
-                    horizontalArrangement = Arrangement.spacedBy(6.dp),
-                    verticalArrangement = Arrangement.spacedBy(4.dp),
-                    modifier = Modifier.fillMaxWidth()
-                ) {
-                    calculatorTransceivers.forEach { radio ->
-                        FilterChip(
-                            selected = radio.uuid == selectedTransceiver.uuid,
-                            onClick = {
-                                if (radio.uuid != selectedUuid) onAction(RadarAction.SelectTransmitter(radio.uuid))
-                            },
-                            label = {
-                                Text(
-                                    text = transceiverTitle(radio),
-                                    maxLines = 1,
-                                    overflow = TextOverflow.Ellipsis
-                                )
-                            }
-                        )
-                    }
+            FlowRow(
+                horizontalArrangement = Arrangement.spacedBy(6.dp),
+                verticalArrangement = Arrangement.spacedBy(4.dp),
+                modifier = Modifier.fillMaxWidth()
+            ) {
+                calculatorTransceivers.forEach { radio ->
+                    FilterChip(
+                        selected = radio.uuid == selectedTransceiver.uuid,
+                        onClick = {
+                            if (radio.uuid != selectedUuid) onAction(RadarAction.SelectTransmitter(radio.uuid))
+                        },
+                        label = {
+                            Text(
+                                text = transceiverTitle(radio),
+                                maxLines = 1,
+                                overflow = TextOverflow.Ellipsis
+                            )
+                        }
+                    )
                 }
             }
         }
 
-        item {
-            DopplerFrequencyCalculator(
-                transponder = selectedTransceiver,
-                orbitalPos = orbitalPos,
-                modifier = Modifier.fillMaxWidth()
-            )
-        }
+        DopplerFrequencyCalculator(
+            transponder = selectedTransceiver,
+            orbitalPos = orbitalPos,
+            modifier = Modifier.fillMaxWidth()
+        )
 
-        item {
-            HorizontalDivider(
-                thickness = 1.dp,
-                color = MaterialTheme.colorScheme.outline.copy(alpha = 0.3f)
-            )
-        }
+        HorizontalDivider(
+            thickness = 1.dp,
+            color = MaterialTheme.colorScheme.outline.copy(alpha = 0.3f)
+        )
 
-        item {
-            CwDecoderPanel(
-                cw = cw,
-                onAction = onAction,
-                requestMicPermission = requestMicPermission,
-                modifier = Modifier.fillMaxWidth()
-            )
-        }
+        CwDecoderPanel(
+            cw = cw,
+            onAction = onAction,
+            requestMicPermission = requestMicPermission,
+            modifier = Modifier.fillMaxWidth()
+        )
     }
 }
 
@@ -555,11 +553,21 @@ private fun DopplerFrequencyCalculator(
 ) {
     if (orbitalPos == null || !DopplerFrequencyCalculator.isLinearTransponder(transponder)) return
 
-    var lastEditedField by remember { mutableStateOf(EditedField.TX) }
-    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 lastEditedField by rememberSaveable(transponder.uuid) { mutableStateOf(EditedField.TX) }
+    var txFrequencyHz by rememberSaveable(transponder.uuid) { mutableStateOf(0L) }
+    var rxFrequencyHz by rememberSaveable(transponder.uuid) { mutableStateOf(0L) }
+    var passbandPosition by rememberSaveable(transponder.uuid) { mutableStateOf(0.5f) }
+    // 每个卫星独立记忆 offset：以 NORAD 编号为 key，存到应用 SharedPreferences
+    val context = LocalContext.current
+    val catnum = transponder.catnum
+    var offsetKHz by remember(catnum) {
+        mutableStateOf(
+            catnum?.let { cn ->
+                context.getSharedPreferences(context.packageName + "_preferences", Context.MODE_PRIVATE)
+                    .getString("offset_khz_$cn", "") ?: ""
+            } ?: ""
+        )
+    }
     var stepSizeKHz by remember { mutableIntStateOf(1) }
 
     val offsetHz = offsetKHz.toDoubleOrNull()?.let { it * 1000 }?.toLong() ?: 0L
@@ -742,7 +750,15 @@ private fun DopplerFrequencyCalculator(
                 ) {
                     BasicTextField(
                         value = offsetKHz,
-                        onValueChange = { offsetKHz = it },
+                        onValueChange = { newValue ->
+                            offsetKHz = newValue
+                            catnum?.let { cn ->
+                                context.getSharedPreferences(
+                                    context.packageName + "_preferences",
+                                    Context.MODE_PRIVATE
+                                ).edit().putString("offset_khz_$cn", newValue).apply()
+                            }
+                        },
                         singleLine = true,
                         textStyle = TextStyle(
                             fontSize = 16.sp,