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/features/security/presentation/pages/security_page.dart

@@ -11,11 +11,15 @@ class SecurityPage extends ConsumerWidget {
   const SecurityPage({
     super.key,
     this.nav = SecurityNav.users,
+    this.routerState,
   });
 
   /// The current nav item to display.
   final SecurityNav nav;
 
+  /// Router state for URL deep-linking (query params).
+  final GoRouterState? routerState;
+
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final colorScheme = Theme.of(context).colorScheme;
@@ -45,7 +49,7 @@ class SecurityPage extends ConsumerWidget {
           ),
           // Section content
           Expanded(
-            child: _SectionContent(nav: nav),
+            child: _SectionContent(nav: nav, routerState: routerState),
           ),
         ],
       ),
@@ -55,17 +59,18 @@ class SecurityPage extends ConsumerWidget {
 
 /// Renders content for the selected nav item.
 class _SectionContent extends StatelessWidget {
-  const _SectionContent({required this.nav});
+  const _SectionContent({required this.nav, this.routerState});
 
   final SecurityNav nav;
+  final GoRouterState? routerState;
 
   @override
   Widget build(BuildContext context) {
     switch (nav) {
       case SecurityNav.users:
-        return const UsersListPage();
+        return UsersListPage(routerState: routerState);
       case SecurityNav.groups:
-        return const GroupsListPage();
+        return GroupsListPage(routerState: routerState);
       default:
         return _PlaceholderSection(nav: nav);
     }