feat(map): mark a station you want to contact in unworked grids

- New MarkedStation model + ISettingsRepo/SettingsRepo persistence
  (single JSON key markedGridStations, org.json only)
- MapState/MapViewModel: markedGrids state, Set/RemoveMarkedStation
  actions; marks auto-clear once the grid becomes worked
- MaidenheadGridOverlay: red fill (alpha 90, same as green worked
  fill) for marked grids at both zoom levels; first-call label mode
  shows the marked callsign in pale yellow on red cells
- MapScreen: unworked-grid dialog gains a 'mark a station you want
  to contact' button + callsign input dialog; marked row shows red
  callsign, 'Marked' in the count column, marking date, delete icon
- i18n: values/ + values-zh/ (grid_mark_*)

core/data/src/main/java/com/rtbishop/look4sat/core/data/repository/SettingsRepo.kt

@@ -269,6 +269,43 @@ class SettingsRepo(
         preferences.edit { putString(keyRoamedGrids, array.toString()) }
     }
 
+    // Marked stations in unworked gridsquares, persisted as a single JSON
+    // object: {"OL62":{"c":call,"t":epochMs}}. One mark per grid.
+    private val keyMarkedGridStations = "markedGridStations"
+
+    override fun getMarkedGridStations(): Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation> {
+        val json = preferences.getString(keyMarkedGridStations, null).orEmpty()
+        if (json.isBlank()) return emptyMap()
+        return try {
+            val root = org.json.JSONObject(json)
+            val result = mutableMapOf<String, com.rtbishop.look4sat.core.domain.model.MarkedStation>()
+            for (grid in root.keys()) {
+                val o = root.optJSONObject(grid) ?: continue
+                val call = o.optString("c").ifBlank { continue }
+                result[grid] = com.rtbishop.look4sat.core.domain.model.MarkedStation(
+                    call = call,
+                    epochMs = o.optLong("t")
+                )
+            }
+            result
+        } catch (_: Exception) {
+            emptyMap()
+        }
+    }
+
+    override fun setMarkedGridStations(stations: Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation>) {
+        val root = org.json.JSONObject()
+        for ((grid, station) in stations) {
+            root.put(
+                grid,
+                org.json.JSONObject()
+                    .put("c", station.call)
+                    .put("t", station.epochMs)
+            )
+        }
+        preferences.edit { putString(keyMarkedGridStations, root.toString()) }
+    }
+
     // LoTW credentials (stored locally on the device only)
     private val keyLoTWCall = "lotwCallsign"
     private val keyLoTWPass = "lotwPassword"

core/data/src/test/java/com/rtbishop/look4sat/core/data/repository/DatabaseRepoTest.kt

@@ -312,6 +312,10 @@ private class FakeSettingsRepo(dataSources: DataSourcesSettings = defaultDataSou
     override fun setRoamedGrids(grids: Set<String>) {
         roamedGrids.value = grids
     }
+
+    override fun getMarkedGridStations(): Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation> = emptyMap()
+
+    override fun setMarkedGridStations(stations: Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation>) = Unit
 }
 
 private fun defaultDataSourcesSettings(): DataSourcesSettings {

core/data/src/test/java/com/rtbishop/look4sat/core/data/repository/SelectionRepoSearchTest.kt

@@ -189,6 +189,8 @@ private class FakeSettingsRepoForSearch : ISettingsRepo {
     override fun setWorkedGridQsos(qsos: Map<String, List<com.rtbishop.look4sat.core.domain.model.GridQso>>) = Unit
     override fun getRoamedGrids(): Set<String> = emptySet()
     override fun setRoamedGrids(grids: Set<String>) = Unit
+    override fun getMarkedGridStations(): Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation> = emptyMap()
+    override fun setMarkedGridStations(stations: Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation>) = Unit
     override fun updateLoTWSettings(settings: com.rtbishop.look4sat.core.domain.model.LoTWSettings) = Unit
     override fun getLastLotwSyncDate(): String = ""
     override fun setLastLotwSyncDate(date: String) = Unit