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

build-logic/convention/src/main/java/com/rtbishop/look4sat/convention/ApplicationPlugin.kt

@@ -35,6 +35,7 @@ internal class ApplicationPlugin : Plugin<Project> {
             implementation(project(":feature:mutual"))
             implementation(project(":feature:passes"))
             implementation(project(":feature:radar"))
+            implementation(project(":feature:roaming"))
             implementation(project(":feature:satellites"))
             implementation(project(":feature:settings"))
             implementation(libs.androidx.core.splashscreen)