feat: add 'current exact position' button for station A

Removed the automatic posA=stationPos override. Added a button
'当前精确位置' below the grid input that fills in the exact station
position from settings (lat/lon + grid). Users can now freely edit
the position fields and use the button when they want the exact
position.

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

@@ -129,6 +129,7 @@ fun MutualScreen(
             onStationBLon = viewModel::onStationBLon,
             onStationBGrid = viewModel::onStationBGrid,
             onStationBMinElev = viewModel::onStationBMinElev,
+            onUseCurrentPosition = viewModel::onUseCurrentPosition,
             onHoursAhead = viewModel::onHoursAhead,
             onClearError = viewModel::clearError
         )
@@ -150,6 +151,7 @@ private fun MutualContent(
     onStationBLon: (String) -> Unit,
     onStationBGrid: (String) -> Unit,
     onStationBMinElev: (Double) -> Unit,
+    onUseCurrentPosition: () -> Unit,
     onHoursAhead: (Int) -> Unit,
     onClearError: () -> Unit
 ) {
@@ -224,6 +226,12 @@ private fun MutualContent(
                         singleLine = true,
                         modifier = Modifier.fillMaxWidth()
                     )
+                    OutlinedButton(
+                        onClick = onUseCurrentPosition,
+                        modifier = Modifier.align(Alignment.End)
+                    ) {
+                        Text("当前精确位置", style = MaterialTheme.typography.bodySmall)
+                    }
                     Text(
                         text = "最小仰角：${state.stationAMinElev.toInt()}°",
                         style = MaterialTheme.typography.bodySmall

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

@@ -139,6 +139,14 @@ class MutualViewModel(
     }
     fun onStationAMinElev(value: Double) = _uiState.update { it.copy(stationAMinElev = value) }
     fun onStationBMinElev(value: Double) = _uiState.update { it.copy(stationBMinElev = value) }
+    fun onUseCurrentPosition() {
+        val pos = settingsRepo.stationPosition.value
+        _uiState.update { it.copy(
+            stationALat = "%.4f".format(pos.latitude),
+            stationALon = "%.4f".format(pos.longitude),
+            stationAGrid = latLonToGrid(pos.latitude, pos.longitude)
+        ) }
+    }
     fun onHoursAhead(value: Int) = _uiState.update { it.copy(hoursAhead = value) }
     fun onSelectPass(index: Int) = _uiState.update { it.copy(selectedPassIndex = index) }
 
@@ -154,19 +162,6 @@ class MutualViewModel(
             return
         }
 
-        // When station A's grid matches the pre-filled grid (user hasn't changed it),
-        // use the exact station position from settings so the pass times and elevation
-        // curves are consistent with the main Passes page.
-        val stationPos = settingsRepo.stationPosition.value
-        val prefillGrid = latLonToGrid(stationPos.latitude, stationPos.longitude)
-        if (state.stationAGrid.trim().uppercase() == prefillGrid) {
-            posA = stationPos
-        }
-        // Same for station B: if its grid matches the user's station, use exact position.
-        if (state.stationBGrid.trim().uppercase() == prefillGrid) {
-            posB = stationPos
-        }
-
         val satellites = satelliteRepo.satellites.value
         if (satellites.isEmpty()) {
             _uiState.update { it.copy(errorMessage = "没有卫星数据，请先在卫星列表中选择卫星") }