fix: preserve calculator frequencies 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
@@ -66,6 +67,7 @@ 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
@@ -190,7 +192,7 @@ fun CalculatorPage(
             }
         }
 
-        item {
+        item(key = "calculator_${selectedTransceiver.uuid}") {
             DopplerFrequencyCalculator(
                 transponder = selectedTransceiver,
                 orbitalPos = orbitalPos,
@@ -555,11 +557,21 @@ 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 +754,15 @@ 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,