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/control_room/npm/presentation/pages/proxy_hosts_page.dart

@@ -1,8 +1,12 @@
+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/features/control_room/npm/presentation/pages/proxy_host_page.dart';
 import 'package:tatlock_ui/features/control_room/npm/presentation/providers/proxy_hosts_provider.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';
 
@@ -33,7 +37,10 @@ class DomainData {
 
 /// Page displaying the list of proxy hosts (domains) from NPM.
 class ProxyHostsPage extends ConsumerStatefulWidget {
-  const ProxyHostsPage({super.key});
+  const ProxyHostsPage({super.key, this.routerState});
+
+  /// Router state for URL deep-linking.
+  final GoRouterState? routerState;
 
   @override
   ConsumerState<ProxyHostsPage> createState() => _ProxyHostsPageState();
@@ -42,10 +49,18 @@ class ProxyHostsPage extends ConsumerStatefulWidget {
 class _ProxyHostsPageState extends ConsumerState<ProxyHostsPage> {
   late final StateNotifierProvider<DataGridController<DomainData>,
       DataGridState<DomainData>> _gridProvider;
+  late PageUrlState _urlState;
+  Timer? _urlSyncTimer;
 
   @override
   void initState() {
     super.initState();
+
+    // Parse URL state
+    _urlState = PageUrlState.fromQueryParams(
+      widget.routerState?.uri.queryParameters ?? {},
+    );
+
     final dio = ref.read(coreApiClientProvider);
     final source = CoreApiDataSource<DomainData>(
       dio: dio,
@@ -53,17 +68,80 @@ class _ProxyHostsPageState extends ConsumerState<ProxyHostsPage> {
       fromJson: DomainData.fromJson,
     );
 
+    // Initialize grid with URL state
     _gridProvider = dataGridProvider<DomainData>(
       source: source,
       config: _buildConfig(),
       idSelector: (d) => d.proxyHostId.toString(),
+      initialSearch: _urlState.search,
+      initialSortColumnIndex: _columnIndexForId(_urlState.sortColumn),
+      initialSortDescending: _urlState.sortDescending,
     );
+
+    // Open document from URL if id present
+    if (_urlState.id != null) {
+      final id = int.tryParse(_urlState.id!);
+      if (id != null) {
+        WidgetsBinding.instance.addPostFrameCallback((_) {
+          ref.read(selectedProxyHostProvider.notifier).select(id);
+        });
+      }
+    }
+  }
+
+  @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 selectedId = ref.read(selectedProxyHostProvider);
+
+    final params = PageUrlState(
+      id: selectedId?.toString(),
+      search: state.searchQuery.isEmpty ? null : state.searchQuery,
+      sortColumn: state.sortColumnIndex != null
+          ? _columnIdForIndex(state.sortColumnIndex!)
+          : null,
+      sortDescending: state.sortDescending,
+    ).toQueryParams();
+
+    updateBrowserUrlParams(params);
   }
 
   DataGridConfig<DomainData> _buildConfig() {
     return DataGridConfig<DomainData>(
       columns: [
         DataGridColumn<DomainData>(
+          id: 'domain',
           header: 'Domain',
           valueBuilder: (d) => d.domain,
           sortable: true,
@@ -72,12 +150,14 @@ class _ProxyHostsPageState extends ConsumerState<ProxyHostsPage> {
           cellBuilder: (context, d) => _DomainCell(domain: d),
         ),
         DataGridColumn<DomainData>(
+          id: 'service',
           header: 'Service',
           valueBuilder: (d) => d.service,
           width: const DataGridColumnWidth.flex(1),
           cellBuilder: (context, d) => _ServiceCell(service: d.service),
         ),
         DataGridColumn<DomainData>(
+          id: 'ssl',
           header: 'SSL',
           valueBuilder: (d) => d.sslEnabled ? 'Enabled' : 'Disabled',
           width: const DataGridColumnWidth.fixed(100),
@@ -101,6 +181,8 @@ class _ProxyHostsPageState extends ConsumerState<ProxyHostsPage> {
 
   void _viewDomain(DomainData domain) {
     ref.read(selectedProxyHostProvider.notifier).select(domain.proxyHostId);
+    // Navigate with id to enable back button
+    context.go('/control-room/proxy-hosts?id=${domain.proxyHostId}');
   }
 
   void _createNew() {
@@ -112,6 +194,15 @@ class _ProxyHostsPageState extends ConsumerState<ProxyHostsPage> {
     final selectedId = ref.watch(selectedProxyHostProvider);
     final isCreating = ref.watch(creatingProxyHostProvider);
 
+    // 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();
+      }
+    });
+
     // Show create page if creating new
     if (isCreating) {
       return ProxyHostPage.create(
@@ -128,6 +219,8 @@ class _ProxyHostsPageState extends ConsumerState<ProxyHostsPage> {
         proxyHostId: selectedId,
         onClose: () {
           ref.read(selectedProxyHostProvider.notifier).clear();
+          // Clear id from URL
+          context.go('/control-room/proxy-hosts');
           ref.read(_gridProvider.notifier).refresh();
         },
       );