feat(map): multiple marked stations per grid with pin-to-top

- MarkedStation storage upgraded to an ordered list per grid
  (Map<String, List<MarkedStation>>); JSON array shape with legacy
  v14.1 single-object migration on read
- MapAction: SetMarkedStation appends (duplicate callsign ignored),
  RemoveMarkedStation deletes by callsign, PinMarkedStation moves a
  mark to the top of the list
- MaidenheadGridOverlay: red fill for any marked grid unchanged;
  first-call label mode shows the FIRST (pinned) marked callsign
- MapScreen: one row per mark (pale-yellow callsign), pin icon
  (冖 + stroke ↑, drawn in-app) left of the Marked label — shown only
  when the grid has 2+ marks; pinned row's icon grey, others pale
  yellow; delete icon per row
- New ic_pin.xml vector drawable; strings grid_mark_pin

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

@@ -0,0 +1,20 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <!-- 冖 (秃宝盖): 顶横 + 两侧短竖, 笔画型 -->
+    <path
+        android:pathData="M5,4 L19,4 M5,4 L5,8.5 M19,4 L19,8.5"
+        android:strokeColor="@android:color/white"
+        android:strokeWidth="2"
+        android:strokeLineCap="round"
+        android:fillColor="#00000000" />
+    <!-- 笔画型向上箭头 ↑: 竖杆 + 两斜线箭头头 -->
+    <path
+        android:pathData="M12,9 L12,20 M12,9 L7.5,14 M12,9 L16.5,14"
+        android:strokeColor="@android:color/white"
+        android:strokeWidth="2"
+        android:strokeLineCap="round"
+        android:fillColor="#00000000" />
+</vector>

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

@@ -137,6 +137,7 @@
     <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="grid_mark_pin">置顶</string>
     <string name="map_visible">日照中</string>
 
     <!-- Settings screen -->

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

@@ -157,6 +157,7 @@
     <string name="grid_mark_confirm">Mark</string>
     <string name="grid_mark_label">Marked</string>
     <string name="grid_mark_delete">Remove mark</string>
+    <string name="grid_mark_pin">Pin to top</string>
     <string name="map_visible">Visible</string>
 
     <!-- Settings screen -->