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_column.dart

@@ -45,6 +45,7 @@ enum DataGridColumnAlignment {
 /// Column definition for DataGrid.
 class DataGridColumn<T> {
   const DataGridColumn({
+    required this.id,
     required this.header,
     required this.valueBuilder,
     this.cellBuilder,
@@ -58,6 +59,9 @@ class DataGridColumn<T> {
     this.tooltip,
   });
 
+  /// Unique identifier for this column (used in URL deep-linking).
+  final String id;
+
   /// Column header text.
   final String header;