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/domain/src/main/java/com/rtbishop/look4sat/core/domain/model/MarkedStation.kt

@@ -0,0 +1,31 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.core.domain.model
+
+/**
+ * A station the user marked in a gridsquare they have NOT yet worked — a
+ * reminder to try to contact that callsign. Stored per 4-char grid (one mark
+ * per grid).
+ *
+ * @param call      the callsign the user wants to work (uppercase)
+ * @param epochMs   the moment the mark was created, UTC milliseconds
+ */
+data class MarkedStation(
+    val call: String,
+    val epochMs: Long
+)

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/repository/ISettingsRepo.kt

@@ -105,6 +105,11 @@ interface ISettingsRepo {
     /** Distinct 4-char gridsquares the account operated from (LoTW <MY_GRIDSQUARE>). */
     fun getRoamedGrids(): Set<String>
     fun setRoamedGrids(grids: Set<String>)
+    /** Stations the user marked as "want to contact" in unworked gridsquares
+     *  (grid -> mark). One mark per grid; cleared automatically when the grid
+     *  becomes worked. */
+    fun getMarkedGridStations(): Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation>
+    fun setMarkedGridStations(stations: Map<String, com.rtbishop.look4sat.core.domain.model.MarkedStation>)
     //endregion
 
     //region # LoTW confirmed-grids settings

core/domain/src/test/java/com/rtbishop/look4sat/core/domain/repository/LoTWGridSyncTest.kt

@@ -21,6 +21,7 @@ import com.rtbishop.look4sat.core.domain.model.DatabaseState
 import com.rtbishop.look4sat.core.domain.model.DataSourcesSettings
 import com.rtbishop.look4sat.core.domain.model.GridQso
 import com.rtbishop.look4sat.core.domain.model.LoTWSettings
+import com.rtbishop.look4sat.core.domain.model.MarkedStation
 import com.rtbishop.look4sat.core.domain.model.OtherSettings
 import com.rtbishop.look4sat.core.domain.model.PassesSettings
 import com.rtbishop.look4sat.core.domain.model.RCSettings
@@ -261,6 +262,9 @@ class LoTWGridSyncTest {
         override fun setWorkedGridQsos(qsos: Map<String, List<GridQso>>) { workedGridQsos = qsos }
         override fun getRoamedGrids(): Set<String> = roamedGrids
         override fun setRoamedGrids(grids: Set<String>) { roamedGrids = grids }
+        private var markedGridStations: Map<String, MarkedStation> = emptyMap()
+        override fun getMarkedGridStations(): Map<String, MarkedStation> = markedGridStations
+        override fun setMarkedGridStations(stations: Map<String, MarkedStation>) { markedGridStations = stations }
         override fun getLastLotwSyncDate(): String = lastSyncDate
         override fun setLastLotwSyncDate(date: String) { lastSyncDate = date }
         override fun getLastLotwSyncCallsign(): String = lastSyncCallsign