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

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

core/presentation/src/main/res/values-zh/strings.xml

@@ -131,6 +131,12 @@
     <string name="grid_qso_first">首通</string>
     <string name="grid_qso_no_sat">卫星未知</string>
     <string name="grid_qso_match">匹配</string>
+    <!-- 未通联网格中的“标记想通联的电台”提醒 -->
+    <string name="grid_mark_station">标记你想通联的电台</string>
+    <string name="grid_mark_callsign">呼号</string>
+    <string name="grid_mark_confirm">标记</string>
+    <string name="grid_mark_label">标记</string>
+    <string name="grid_mark_delete">删除标记</string>
     <string name="map_visible">日照中</string>
 
     <!-- Settings screen -->

core/presentation/src/main/res/values/strings.xml

@@ -151,6 +151,12 @@
     <string name="grid_qso_first">First</string>
     <string name="grid_qso_no_sat">satellite unknown</string>
     <string name="grid_qso_match">Match</string>
+    <!-- Mark-a-station reminder in unworked grids -->
+    <string name="grid_mark_station">Mark a station you want to contact</string>
+    <string name="grid_mark_callsign">Callsign</string>
+    <string name="grid_mark_confirm">Mark</string>
+    <string name="grid_mark_label">Marked</string>
+    <string name="grid_mark_delete">Remove mark</string>
     <string name="map_visible">Visible</string>
 
     <!-- Settings screen -->