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/shared/components/data_grid/data_grid_state.dart

@@ -31,7 +31,7 @@ sealed class DataGridState<T> with _$DataGridState<T> {
     @Default(false) bool sortDescending,
 
     /// Currently selected item IDs (if selectable).
-    @Default({}) Set<Object> selectedIds,
+    @Default({}) Set<String> selectedIds,
 
     /// Current page (for paginated mode).
     @Default(0) int currentPage,