feat: bidirectional drag sync between elevation curve and radar track

- ElevationCurveChart is now controlled (progress + onProgressChange)
- MutualRadarView shows shared time-cursor positions for both stations
  and is draggable/tappable to move the cursor
- MutualPassCard owns a single dragProgress feeding both charts, so
  dragging either chart moves the other in sync

feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/ElevationCurveChart.kt

@@ -30,10 +30,7 @@ import androidx.compose.foundation.layout.padding
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
-import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.graphics.toArgb
@@ -50,6 +47,8 @@ import kotlin.math.roundToInt
 
 /**
  * Draggable dual-station elevation curve chart.
+ * Controlled component: [progress] (0..1) and [onProgressChange] are owned by the parent,
+ * so the same time cursor can be shared with the radar track view.
  */
 @Composable
 fun ElevationCurveChart(
@@ -57,6 +56,8 @@ fun ElevationCurveChart(
     startTime: Long,
     endTime: Long,
     maxElev: Double,
+    progress: Float = 0.5f,
+    onProgressChange: (Float) -> Unit = {},
     modifier: Modifier = Modifier
 ) {
     if (samples.isEmpty()) return
@@ -73,8 +74,6 @@ fun ElevationCurveChart(
     val gridColorArgb = gridColor.toArgb()
     val timeFormat = remember { SimpleDateFormat("HH:mm", Locale.getDefault()) }
 
-    var dragProgress by remember { mutableFloatStateOf(0.5f) }
-
     Column(modifier = modifier.fillMaxWidth()) {
         Canvas(
             modifier = Modifier
@@ -83,12 +82,12 @@ fun ElevationCurveChart(
                 .background(MaterialTheme.colorScheme.surfaceVariant.copy(alpha = 0.3f))
                 .pointerInput(Unit) {
                     detectTapGestures { offset ->
-                        dragProgress = (offset.x / size.width.toFloat()).coerceIn(0f, 1f)
+                        onProgressChange((offset.x / size.width.toFloat()).coerceIn(0f, 1f))
                     }
                 }
                 .pointerInput(Unit) {
-                    detectDragGestures { _, dragAmount ->
-                        dragProgress = (dragProgress + dragAmount.x / size.width.toFloat()).coerceIn(0f, 1f)
+                    detectDragGestures { change, _ ->
+                        onProgressChange((change.position.x / size.width.toFloat()).coerceIn(0f, 1f))
                     }
                 }
         ) {
@@ -180,8 +179,8 @@ fun ElevationCurveChart(
             drawPath(pathB, colorB, style = Stroke(width = 2.5f))
 
             // Drag indicator
-            val dragX = plotLeft + dragProgress * plotWidth
-            val dragTime = startTime + (dragProgress * (endTime - startTime)).toLong()
+            val dragX = plotLeft + progress * plotWidth
+            val dragTime = startTime + (progress * (endTime - startTime)).toLong()
 
             drawLine(
                 onSurfaceColor,

feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualRadarView.kt

@@ -18,6 +18,8 @@
 package com.rtbishop.look4sat.feature.mutual
 
 import androidx.compose.foundation.Canvas
+import androidx.compose.foundation.gestures.detectDragGestures
+import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
@@ -34,6 +36,7 @@ import androidx.compose.ui.graphics.Path
 import androidx.compose.ui.graphics.PathEffect
 import androidx.compose.ui.graphics.drawscope.DrawScope
 import androidx.compose.ui.graphics.drawscope.Stroke
+import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.text.TextMeasurer
 import androidx.compose.ui.text.TextStyle
 import androidx.compose.ui.text.drawText
@@ -50,10 +53,14 @@ private const val STROKE_WIDTH = 2.5f
 /**
  * Polar radar chart showing the satellite track as seen from both stations
  * on one plot. Station A is drawn solid, station B dashed.
+ * Controlled: [progress] (0..1) and [onProgressChange] are owned by the parent
+ * so the time cursor can be shared with the elevation curve chart.
  */
 @Composable
 fun MutualRadarView(
     trackSamples: List<TrackSample>,
+    progress: Float = 0.5f,
+    onProgressChange: (Float) -> Unit = {},
     labelA: String = "站点A",
     labelB: String = "站点B",
     modifier: Modifier = Modifier
@@ -66,12 +73,29 @@ fun MutualRadarView(
     val textColor = MaterialTheme.colorScheme.onSurfaceVariant
     val measurer = rememberTextMeasurer()
 
+    // Current-time samples (both stations' position at the shared cursor)
+    val t0 = trackSamples.first().time
+    val t1 = trackSamples.last().time
+    val selectedTime = t0 + ((t1 - t0) * progress).toLong()
+    val visibleSamples = trackSamples.filter { it.time <= selectedTime }
+    val currentSample = visibleSamples.lastOrNull() ?: trackSamples.first()
+
     Column(modifier = modifier.fillMaxWidth()) {
         Canvas(
             modifier = Modifier
                 .fillMaxWidth()
                 .aspectRatio(1f)
                 .padding(4.dp)
+                .pointerInput(Unit) {
+                    detectTapGestures { offset ->
+                        onProgressChange((offset.x / size.width.toFloat()).coerceIn(0f, 1f))
+                    }
+                }
+                .pointerInput(Unit) {
+                    detectDragGestures { change, _ ->
+                        onProgressChange((change.position.x / size.width.toFloat()).coerceIn(0f, 1f))
+                    }
+                }
         ) {
             val radius = size.minDimension / 2f * 0.92f
             val center = Offset(size.width / 2f, size.height / 2f)
@@ -125,6 +149,14 @@ fun MutualRadarView(
             if (last.elevationA > 0f) drawCircle(colorA, 5f, losA)
             if (last.elevationB > 0f) drawCircle(colorB, 5f, losB)
 
+            // Shared time-cursor positions (both stations at selectedTime)
+            val cursorA = sph2Cart(center, currentSample.azimuthA, currentSample.elevationA, radius)
+            val cursorB = sph2Cart(center, currentSample.azimuthB, currentSample.elevationB, radius)
+            drawCircle(colorA, 14f, cursorA, style = Stroke(3f))
+            drawCircle(colorA, 6f, cursorA)
+            drawCircle(colorB, 14f, cursorB, style = Stroke(3f))
+            drawCircle(colorB, 6f, cursorB)
+
             // Center dot
             drawCircle(gridColor, 4f, center)
         }

feature/mutual/src/main/java/com/rtbishop/look4sat/feature/mutual/MutualScreen.kt

@@ -48,7 +48,9 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.res.painterResource
@@ -361,6 +363,9 @@ private fun MutualPassCard(
     onClick: () -> Unit,
     onNavigateToRadar: () -> Unit
 ) {
+    // Shared time cursor: both the elevation curve and the radar track view
+    // are controlled by this single progress value for bidirectional drag sync.
+    var dragProgress by remember(pass) { mutableFloatStateOf(0.5f) }
     ElevatedCard(
         modifier = Modifier
             .fillMaxWidth()
@@ -419,6 +424,8 @@ private fun MutualPassCard(
                         )
                         MutualRadarView(
                             trackSamples = pass.trackSamples,
+                            progress = dragProgress,
+                            onProgressChange = { dragProgress = it },
                             labelA = "你",
                             labelB = "友台"
                         )
@@ -429,7 +436,9 @@ private fun MutualPassCard(
                             samples = pass.elevationSamples,
                             startTime = pass.startTime,
                             endTime = pass.endTime,
-                            maxElev = maxOf(pass.maxElevationA, pass.maxElevationB, 10.0)
+                            maxElev = maxOf(pass.maxElevationA, pass.maxElevationB, 10.0),
+                            progress = dragProgress,
+                            onProgressChange = { dragProgress = it }
                         )
                     }
                     Spacer(Modifier.height(8.dp))