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/groups/groups_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';
 
@@ -31,7 +35,10 @@ class GroupData {
 
 /// Groups list page using the shared DataGrid component.
 class GroupsListPage extends ConsumerStatefulWidget {
-  const GroupsListPage({super.key});
+  const GroupsListPage({super.key, this.routerState});
+
+  /// Router state for URL deep-linking.
+  final GoRouterState? routerState;
 
   @override
   ConsumerState<GroupsListPage> createState() => _GroupsListPageState();
@@ -40,11 +47,19 @@ class GroupsListPage extends ConsumerStatefulWidget {
 class _GroupsListPageState extends ConsumerState<GroupsListPage> {
   late final StateNotifierProvider<DataGridController<GroupData>,
       DataGridState<GroupData>> _gridProvider;
+  late PageUrlState _urlState;
+  Timer? _urlSyncTimer;
   bool _isSyncing = false;
 
   @override
   void initState() {
     super.initState();
+
+    // Parse URL state
+    _urlState = PageUrlState.fromQueryParams(
+      widget.routerState?.uri.queryParameters ?? {},
+    );
+
     // Create provider in initState to ensure stable reference
     final dio = ref.read(coreApiClientProvider);
     final source = CoreApiDataSource<GroupData>(
@@ -53,13 +68,63 @@ class _GroupsListPageState extends ConsumerState<GroupsListPage> {
       fromJson: GroupData.fromJson,
     );
 
+    // Initialize grid with URL state
     _gridProvider = dataGridProvider<GroupData>(
       source: source,
       config: _buildConfig(),
       idSelector: (g) => g.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);
@@ -96,6 +161,7 @@ class _GroupsListPageState extends ConsumerState<GroupsListPage> {
     return DataGridConfig<GroupData>(
       columns: [
         DataGridColumn<GroupData>(
+          id: 'name',
           header: 'Name',
           valueBuilder: (g) => g.name,
           sortable: true,
@@ -103,18 +169,21 @@ class _GroupsListPageState extends ConsumerState<GroupsListPage> {
           width: const DataGridColumnWidth.flex(2),
         ),
         DataGridColumn<GroupData>(
+          id: 'members',
           header: 'Members',
           valueBuilder: (g) => g.memberCount.toString(),
           width: const DataGridColumnWidth.fixed(100),
           alignment: DataGridColumnAlignment.end,
         ),
         DataGridColumn<GroupData>(
+          id: 'type',
           header: 'Type',
           valueBuilder: (g) => g.isSuperuser ? 'Superuser' : 'Standard',
           cellBuilder: (context, g) => _GroupTypeBadge(isSuperuser: g.isSuperuser),
           width: const DataGridColumnWidth.fixed(120),
         ),
         DataGridColumn<GroupData>(
+          id: 'parent',
           header: 'Parent',
           valueBuilder: (g) => g.parentName ?? '-',
           width: const DataGridColumnWidth.flex(1),
@@ -129,6 +198,15 @@ class _GroupsListPageState extends ConsumerState<GroupsListPage> {
 
   @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<GroupData>(
       provider: _gridProvider,
       config: _buildConfig(),