feat(roaming): add Roaming page with 3x3 Maidenhead grid

Ports the QTH定位器 (com.us1pm.gridsquarelocator) location panel into Look4Sat as a new 'Roaming' page, restyled with the app's own look.

UI (top to bottom):
- Info header: GPS status dot, big 8-char locator, Lat/Lon rows with DMS + 5-decimal display, and a GPS 定位 button
- 3x3 grid panel: the current 4-char Maidenhead square (e.g. OL42) centered, surrounded by its 8 neighbors (OL33..OL51), with a red position marker in the center cell

Logic:
- QthConverter gains qthNeighbors(square) building the 3x3 grid with field/square carry at boundaries (AA00 wraps to RR99, IO91 crosses into J field), and qthToSquare(locator) extracting the 4-char square
- Verified against the decompiled app algorithm and the reference screenshot (OL42 grid matches exactly); 9 unit tests cover normal, boundary and field-wrap cases

Navigation:
- New bottom-nav item 'Roaming' between Match and Settings, with a crosshair icon
- New feature:roaming module (ViewModel + Compose screen) registered in the app

Build config:
- Lowered Gradle JVM heap from -Xmx6g to 768m: the 2GB build server froze on the old value; heavy release builds stay on GitHub Actions

app/src/main/java/com/rtbishop/look4sat/MainScreen.kt

@@ -85,6 +85,8 @@ import com.rtbishop.look4sat.feature.mutual.MutualScreen
 import com.rtbishop.look4sat.feature.mutual.MutualViewModel
 import com.rtbishop.look4sat.feature.passes.PassesDestination
 import com.rtbishop.look4sat.feature.radar.RadarDestination
+import com.rtbishop.look4sat.feature.roaming.RoamingScreen
+import com.rtbishop.look4sat.feature.roaming.RoamingViewModel
 import com.rtbishop.look4sat.feature.satellites.SatellitesDestination
 import com.rtbishop.look4sat.feature.settings.SettingsDestination
 
@@ -130,7 +132,7 @@ fun MainScreen(navigateToRadar: () -> Unit = {}) {
     val currentKey = backStack.lastOrNull()
     val navigateBack: () -> Unit = { backStack.removeLastOrNull() }
     val fadeTransition = fadeIn(animationSpec = tween(350)) togetherWith fadeOut(animationSpec = tween(350))
-    val navItems = listOf(Screen.Satellites, Screen.Passes, Screen.Radar, Screen.Mutual, Screen.Map, Screen.Settings)
+    val navItems = listOf(Screen.Satellites, Screen.Passes, Screen.Radar, Screen.Mutual, Screen.Roaming, Screen.Map, Screen.Settings)
 
     val context = LocalContext.current
     val container = (context.applicationContext as IContainerProvider).getMainContainer()
@@ -224,6 +226,13 @@ fun MainScreen(navigateToRadar: () -> Unit = {}) {
                                 }
                             )
                         }
+                        entry<Screen.Roaming> {
+                            val roamingViewModel: RoamingViewModel = viewModel(
+                                viewModelStoreOwner = context as ViewModelStoreOwner,
+                                factory = RoamingViewModel.factory(container)
+                            )
+                            RoamingScreen(viewModel = roamingViewModel)
+                        }
                         entry<Screen.Settings> {
                             SettingsDestination()
                         }