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/routing/url_state_web.dart

@@ -0,0 +1,13 @@
+import 'package:web/web.dart' as web;
+
+/// Updates browser URL with query parameters without triggering navigation.
+///
+/// Uses browser's replaceState API to update the URL bar for bookmarking
+/// without causing Flutter/GoRouter to rebuild the page.
+void updateBrowserUrlParams(Map<String, String> params) {
+  final currentUri = Uri.parse(web.window.location.href);
+  final newUri = currentUri.replace(
+    queryParameters: params.isEmpty ? null : params,
+  );
+  web.window.history.replaceState(null, '', newUri.toString());
+}