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.dart

@@ -0,0 +1,98 @@
+import 'package:flutter/foundation.dart' show kIsWeb;
+
+// Conditional import for web-only functionality
+import 'url_state_stub.dart' if (dart.library.js_interop) 'url_state_web.dart'
+    as platform;
+
+/// Parses and builds URL query parameters for page state.
+///
+/// Used to enable deep-linking for DataGrid and filter panel state.
+/// Query params are read once on page load and updated via browser
+/// replaceState to avoid GoRouter rebuild loops.
+class PageUrlState {
+  const PageUrlState({
+    this.id,
+    this.filter,
+    this.search,
+    this.sortColumn,
+    this.sortDescending = false,
+  });
+
+  /// Opened document ID (view/edit mode).
+  final String? id;
+
+  /// Filter panel search value.
+  final String? filter;
+
+  /// DataGrid search value.
+  final String? search;
+
+  /// Column ID for sorting.
+  final String? sortColumn;
+
+  /// Sort direction (true = descending).
+  final bool sortDescending;
+
+  /// Parse from query parameters map.
+  factory PageUrlState.fromQueryParams(Map<String, String> params) {
+    return PageUrlState(
+      id: params['id'],
+      filter: params['filter'],
+      search: params['search'],
+      sortColumn: params['sort'],
+      sortDescending: params['order'] == 'desc',
+    );
+  }
+
+  /// Convert to query parameter map (omits empty/default values).
+  Map<String, String> toQueryParams() {
+    return {
+      if (id != null && id!.isNotEmpty) 'id': id!,
+      if (filter != null && filter!.isNotEmpty) 'filter': filter!,
+      if (search != null && search!.isNotEmpty) 'search': search!,
+      if (sortColumn != null && sortColumn!.isNotEmpty) 'sort': sortColumn!,
+      if (sortDescending) 'order': 'desc',
+    };
+  }
+
+  /// Create a copy with modified values.
+  PageUrlState copyWith({
+    String? id,
+    String? filter,
+    String? search,
+    String? sortColumn,
+    bool? sortDescending,
+    bool clearId = false,
+  }) {
+    return PageUrlState(
+      id: clearId ? null : (id ?? this.id),
+      filter: filter ?? this.filter,
+      search: search ?? this.search,
+      sortColumn: sortColumn ?? this.sortColumn,
+      sortDescending: sortDescending ?? this.sortDescending,
+    );
+  }
+
+  /// Whether any state is present.
+  bool get isEmpty =>
+      id == null &&
+      (filter == null || filter!.isEmpty) &&
+      (search == null || search!.isEmpty) &&
+      sortColumn == null;
+
+  @override
+  String toString() =>
+      'PageUrlState(id: $id, filter: $filter, search: $search, '
+      'sort: $sortColumn, desc: $sortDescending)';
+}
+
+/// Updates browser URL with query parameters without triggering navigation.
+///
+/// Uses browser's replaceState API on web, no-op on other platforms.
+/// This allows the URL to stay in sync for bookmarking/sharing without
+/// causing Flutter to rebuild.
+void updateBrowserUrlParams(Map<String, String> params) {
+  if (kIsWeb) {
+    platform.updateBrowserUrlParams(params);
+  }
+}

lib/routing/url_state_stub.dart

@@ -0,0 +1,4 @@
+/// Stub implementation for non-web platforms.
+void updateBrowserUrlParams(Map<String, String> params) {
+  // No-op on non-web platforms
+}

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());
+}