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

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/utility/QthConverter.kt

@@ -100,3 +100,47 @@ private fun isValidPosition(lat: Double, lon: Double): Boolean {
 private fun isValidLocator(locator: String): Boolean {
     return locator.matches("[a-xA-X]{2}\\d{2}[a-xA-X]{2}(?:\\d{2}(?:[a-xA-X]{2})?)?".toRegex())
 }
+
+/**
+ * Returns the 4-char field+square part of a locator, e.g. "OL42ih45" -> "OL42".
+ * A 4-char input is returned as-is when valid.
+ */
+fun qthToSquare(locator: String): String {
+    val upper = locator.trim().uppercase()
+    return when {
+        upper.length >= 4 && isValidLocator(upper) -> upper.take(4)
+        upper.length == 4 && upper.matches("[a-xA-X]{2}\\d{2}".toRegex()) -> upper
+        else -> "----"
+    }
+}
+
+/**
+ * Builds the 3x3 grid of 4-char squares surrounding [square] (e.g. "OL42").
+ * Row 0 = north (lat +1), col 0 = west (lon -1). Handles field/square carry
+ * at boundaries (e.g. "AA00" wraps to "RR99" at the south-west corner).
+ * Mirrors the neighbor logic decompiled from the QTH定位器 app.
+ */
+fun qthNeighbors(square: String): List<String> {
+    if (square.length != 4) return emptyList()
+    val lonField = (square[0].uppercaseChar().code - 65).coerceIn(0, 17)
+    val latField = (square[1].uppercaseChar().code - 65).coerceIn(0, 17)
+    val lonSquare = square[2].digitToCharOrNull() ?: return emptyList()
+    val latSquare = square[3].digitToCharOrNull() ?: return emptyList()
+    val result = mutableListOf<String>()
+    for (dLat in 1 downTo -1) {          // north -> south
+        for (dLon in -1..1) {            // west -> east
+            var lf = lonField
+            var tf = latField
+            var ls = lonSquare + dLon
+            var ts = latSquare + dLat
+            if (ls < 0) { lf -= 1; ls = 9 } else if (ls > 9) { lf += 1; ls = 0 }
+            if (ts < 0) { tf -= 1; ts = 9 } else if (ts > 9) { tf += 1; ts = 0 }
+            lf = (lf + 18) % 18
+            tf = (tf + 18) % 18
+            result += "${('A' + lf).toChar()}${('A' + tf).toChar()}$ls$ts"
+        }
+    }
+    return result
+}
+
+private fun Char.digitToCharOrNull(): Int? = digitToIntOrNull()

core/domain/src/test/java/com/rtbishop/look4sat/core/domain/QthConverterTest.kt

@@ -18,7 +18,9 @@
 package com.rtbishop.look4sat.core.domain
 
 import com.rtbishop.look4sat.core.domain.utility.positionToQth
+import com.rtbishop.look4sat.core.domain.utility.qthNeighbors
 import com.rtbishop.look4sat.core.domain.utility.qthToPosition
+import com.rtbishop.look4sat.core.domain.utility.qthToSquare
 import org.junit.Test
 
 class QthConverterTest {
@@ -82,4 +84,42 @@ class QthConverterTest {
             assert(qth == qth2) { "Roundtrip failed for ($lat, $lon): $qth -> $qth2" }
         }
     }
+
+    @Test
+    fun `Given square returns correct 3x3 neighbors`() {
+        // Reference grid from the QTH定位器 screenshot: OL42
+        val neighbors = qthNeighbors("OL42")
+        assert(neighbors == listOf(
+            "OL33", "OL43", "OL53",
+            "OL32", "OL42", "OL52",
+            "OL31", "OL41", "OL51"
+        )) { "OL42 grid mismatch: $neighbors" }
+        // Center cell must be the input itself
+        assert(neighbors[4] == "OL42")
+        // 9 cells, all distinct
+        assert(neighbors.size == 9 && neighbors.toSet().size == 9)
+    }
+
+    @Test
+    fun `Given boundary square wraps fields correctly`() {
+        // South-west corner: AA00 neighbors wrap to RR99 / RA90 etc.
+        val sw = qthNeighbors("AA00")
+        assert(sw.size == 9 && sw.toSet().size == 9)
+        assert(sw[0] == "RA91" && sw[4] == "AA00" && sw[6] == "RR99" && sw[8] == "AR19")
+        // North-east corner: RR99 wraps to AA00
+        val ne = qthNeighbors("RR99")
+        assert(ne.size == 9 && ne.toSet().size == 9)
+        assert(ne[0] == "RA80" && ne[4] == "RR99" && ne[8] == "AR08")
+        // Field boundary: IO91's east neighbors cross into J field
+        val london = qthNeighbors("IO91")
+        assert(london[2] == "JO02" && london[5] == "JO01")
+    }
+
+    @Test
+    fun `Given full locator returns square part`() {
+        assert(qthToSquare("OL42ih45") == "OL42")
+        assert(qthToSquare("io91VL39FX") == "IO91")
+        assert(qthToSquare("JN58") == "JN58")
+        assert(qthToSquare("garbage!!") == "----")
+    }
 }

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

@@ -38,6 +38,9 @@ sealed class Screen(val iconResId: Int, val titleResId: Int) : NavKey {
     @Serializable
     data object Mutual : Screen(R.drawable.ic_match, R.string.nav_mutual)
 
+    @Serializable
+    data object Roaming : Screen(R.drawable.ic_roaming, R.string.nav_roaming)
+
     @Serializable
     data object Settings : Screen(R.drawable.ic_settings, R.string.nav_prefs)
 }

core/presentation/src/main/res/drawable/ic_roaming.xml

@@ -0,0 +1,25 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="96"
+    android:viewportHeight="96">
+
+    <!-- Crosshair / grid-locator style icon: concentric rings + center dot -->
+    <path
+        android:fillColor="@android:color/transparent"
+        android:strokeColor="@android:color/white"
+        android:strokeWidth="6"
+        android:strokeLineCap="round"
+        android:strokeLineJoin="round"
+        android:pathData="M 48 12 L 48 84 M 12 48 L 84 48" />
+
+    <path
+        android:fillColor="@android:color/transparent"
+        android:strokeColor="@android:color/white"
+        android:strokeWidth="6"
+        android:pathData="M 48 26 A 22 22 0 1 0 48.1 26" />
+
+    <path
+        android:fillColor="@android:color/white"
+        android:pathData="M 44 44 L 52 44 L 52 52 L 44 52 Z" />
+</vector>