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/users/users_list_page.dart

@@ -1,6 +1,10 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
 import 'package:tatlock_ui/core/api/api_client.dart';
+import 'package:tatlock_ui/routing/url_state.dart';
 import 'package:tatlock_ui/shared/components/data_grid/adapters/core_api_source.dart';
 import 'package:tatlock_ui/shared/components/data_grid/data_grid_exports.dart';
 
@@ -61,7 +65,10 @@ class UserData {
 
 /// Users list page using the shared DataGrid component.
 class UsersListPage extends ConsumerStatefulWidget {
-  const UsersListPage({super.key});
+  const UsersListPage({super.key, this.routerState});
+
+  /// Router state for URL deep-linking.
+  final GoRouterState? routerState;
 
   @override
   ConsumerState<UsersListPage> createState() => _UsersListPageState();
@@ -70,11 +77,19 @@ class UsersListPage extends ConsumerStatefulWidget {
 class _UsersListPageState extends ConsumerState<UsersListPage> {
   late final StateNotifierProvider<DataGridController<UserData>,
       DataGridState<UserData>> _gridProvider;
+  late PageUrlState _urlState;
+  Timer? _urlSyncTimer;
   bool _isSyncing = false;
 
   @override
   void initState() {
     super.initState();
+
+    // Parse URL state
+    _urlState = PageUrlState.fromQueryParams(
+      widget.routerState?.uri.queryParameters ?? {},
+    );
+
     final dio = ref.read(coreApiClientProvider);
     final source = CoreApiDataSource<UserData>(
       dio: dio,
@@ -82,13 +97,63 @@ class _UsersListPageState extends ConsumerState<UsersListPage> {
       fromJson: UserData.fromJson,
     );
 
+    // Initialize grid with URL state
     _gridProvider = dataGridProvider<UserData>(
       source: source,
       config: _buildConfig(),
       idSelector: (u) => u.id,
+      initialSearch: _urlState.search,
+      initialSortColumnIndex: _columnIndexForId(_urlState.sortColumn),
+      initialSortDescending: _urlState.sortDescending,
     );
   }
 
+  @override
+  void dispose() {
+    _urlSyncTimer?.cancel();
+    super.dispose();
+  }
+
+  /// Find column index by column ID.
+  int? _columnIndexForId(String? columnId) {
+    if (columnId == null) return null;
+    final columns = _buildConfig().columns;
+    for (var i = 0; i < columns.length; i++) {
+      if (columns[i].id == columnId) return i;
+    }
+    return null;
+  }
+
+  /// Get column ID by index.
+  String? _columnIdForIndex(int index) {
+    final columns = _buildConfig().columns;
+    if (index >= 0 && index < columns.length) {
+      return columns[index].id;
+    }
+    return null;
+  }
+
+  /// Schedule URL sync with debounce.
+  void _scheduleUrlSync() {
+    _urlSyncTimer?.cancel();
+    _urlSyncTimer = Timer(const Duration(milliseconds: 500), _syncUrlParams);
+  }
+
+  /// Sync current state to URL.
+  void _syncUrlParams() {
+    final state = ref.read(_gridProvider);
+
+    final params = PageUrlState(
+      search: state.searchQuery.isEmpty ? null : state.searchQuery,
+      sortColumn: state.sortColumnIndex != null
+          ? _columnIdForIndex(state.sortColumnIndex!)
+          : null,
+      sortDescending: state.sortDescending,
+    ).toQueryParams();
+
+    updateBrowserUrlParams(params);
+  }
+
   Future<void> _syncFromAuthentik() async {
     if (_isSyncing) return;
     setState(() => _isSyncing = true);
@@ -125,6 +190,7 @@ class _UsersListPageState extends ConsumerState<UsersListPage> {
     return DataGridConfig<UserData>(
       columns: [
         DataGridColumn<UserData>(
+          id: 'user',
           header: 'User',
           valueBuilder: (u) => u.name,
           sortable: true,
@@ -133,12 +199,14 @@ class _UsersListPageState extends ConsumerState<UsersListPage> {
           cellBuilder: (context, u) => _UserCell(user: u),
         ),
         DataGridColumn<UserData>(
+          id: 'roles',
           header: 'Roles',
           valueBuilder: (u) => u.roles.join(', '),
           width: const DataGridColumnWidth.flex(1),
           cellBuilder: (context, u) => _RolesCell(roles: u.roles),
         ),
         DataGridColumn<UserData>(
+          id: 'lastLogin',
           header: 'Last Login',
           valueBuilder: (u) => u.lastLoginFormatted,
           width: const DataGridColumnWidth.fixed(120),
@@ -154,6 +222,15 @@ class _UsersListPageState extends ConsumerState<UsersListPage> {
 
   @override
   Widget build(BuildContext context) {
+    // Listen for grid state changes to sync URL
+    ref.listen(_gridProvider, (previous, next) {
+      if (previous?.searchQuery != next.searchQuery ||
+          previous?.sortColumnIndex != next.sortColumnIndex ||
+          previous?.sortDescending != next.sortDescending) {
+        _scheduleUrlSync();
+      }
+    });
+
     return DataGrid<UserData>(
       provider: _gridProvider,
       config: _buildConfig(),