feat: add URL deep-linking for DataGrids

- Add PageUrlState utility for URL ↔ state serialization
- Add column `id` field for unique column identification in URLs
- Update idSelector to return String for URL compatibility
- All DataGrid pages now support URL params: search, sort, order, id
- Browser URL updates via replaceState (no GoRouter rebuilds)
- Add FilterPanelSemantics for filter panel semantic IDs
- Add TESTING.md documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/core/semantics/semantic_ids.dart

@@ -103,6 +103,17 @@ abstract class SettingsSemantics {
   static const defaultRoomDropdown = 'settings_defaultRoom';
 }
 
+/// Filter panel IDs.
+abstract class FilterPanelSemantics {
+  static const panel = 'filterPanel';
+  static const search = 'filterPanel_search';
+  static const searchClear = 'filterPanel_search_clear';
+  static const refresh = 'filterPanel_refresh';
+
+  /// Generate ID for a filter item.
+  static String item(String id) => 'filterPanel_item_$id';
+}
+
 /// Loading/state indicator IDs.
 abstract class StateSemantics {
   static const authLoading = 'state_auth_loading';