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>

docs/ARCHITECTURE.md

@@ -20,6 +20,7 @@ lib/
 │   ├── auth/                     # Authentik OIDC integration
 │   ├── config/                   # Environment configuration
 │   ├── error/                    # Error types and handling
+│   ├── semantics/                # Semantic IDs for automation
 │   └── theme/                    # Material 3 theming
 ├── routing/                      # go_router configuration
 ├── shared/                       # Reusable components
@@ -544,3 +545,28 @@ import 'package:tatlock_ui/features/containers/domain/entities/container.dart';
 // Bad - importing model in presentation
 import '../data/models/container_model.dart'; // Don't do this
 ```
+
+## Semantic Identifiers for Automation
+
+All interactive widgets should have semantic identifiers for UI automation. This enables reliable testing with Puppeteer, Appium, and other automation tools.
+
+### Quick Reference
+
+```dart
+import 'package:tatlock_ui/core/semantics/semantic_ids.dart';
+
+// Wrap interactive widgets with Semantics
+Semantics(
+  identifier: DataGridSemantics.row(item.id),
+  label: 'Select ${item.name}',
+  child: MyRowWidget(item: item),
+)
+```
+
+### Key Points
+
+1. **Central ID Registry** - All IDs defined in `lib/core/semantics/semantic_ids.dart`
+2. **Naming Convention** - `{area}_{component}_{identifier}` (e.g., `dataGrid_row_abc123`)
+3. **Web Enabled** - Semantics tree exposed via `SemanticsBinding.instance.ensureSemantics()` in `main.dart`
+
+For complete documentation on semantic patterns, automation queries, and best practices, see **[TESTING.md](./TESTING.md)**.