feat: add mutual pass query with dual-station elevation curve

Port satlover.de dual-station pass matching feature:
- New feature/mutual module with mutual pass data model
- MutualViewModel: compute overlapping passes for two stations
- ElevationCurveChart: Canvas-based dual elevation curve with drag
- MutualScreen: input form + results with expandable cards
- Navigation: add Mutual tab to bottom navigation bar
- i18n: add Chinese/English strings for new feature

core/presentation/src/main/java/com/rtbishop/look4sat/core/presentation/Navigation.kt

@@ -35,6 +35,9 @@ sealed class Screen(val iconResId: Int, val titleResId: Int) : NavKey {
     @Serializable
     data object Map : Screen(R.drawable.ic_map, R.string.nav_map)
 
+    @Serializable
+    data object Mutual : Screen(R.drawable.ic_radar, R.string.nav_mutual)
+
     @Serializable
     data object Settings : Screen(R.drawable.ic_settings, R.string.nav_prefs)
 }

core/presentation/src/main/res/values-zh/strings.xml

@@ -9,6 +9,7 @@
     <string name="nav_sat">卫星</string>
     <string name="nav_pass">过境</string>
     <string name="nav_radar">雷达</string>
+    <string name="nav_mutual">对台</string>
     <string name="nav_map">地图</string>
     <string name="nav_prefs">设置</string>
 

core/presentation/src/main/res/values/strings.xml

@@ -10,6 +10,7 @@
     <string name="nav_sat">Satellites</string>
     <string name="nav_pass">Passes</string>
     <string name="nav_radar">Radar</string>
+    <string name="nav_mutual">Mutual</string>
     <string name="nav_map">Map</string>
     <string name="nav_prefs">Settings</string>