Add permanent device deletion and refine frontend UI/snackbars

Backend:
- Add db::devices::delete to erase a device and its events atomically
- Expose DELETE /api/devices/{mac}/permanently, wired to OpenAPI
- Cover the new db method and endpoint with tests

Frontend:
- Delete action for not-registered devices (detail screen + list row)
  and an opt-in "permanently delete" checkbox in the Forget dialog
- Navigate to the devices list after deleting from the detail screen
- Refine button emphasis to M3: single filled primary, error-colored
  text buttons for destructive actions, Test demoted to filled-tonal
- Flash the backend-config Test button red on a failed connection test
- Render snackbars through a top-level ScaffoldMessenger host so they
  show above dialogs; keep the built-in SnackBar (with an Overlay host)

Docs:
- CLAUDE.md: rustfmt edition 2024, don't revert formatter-only changes,
  prefer built-in Flutter components, follow existing patterns + M3

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/utils/api/oott_api_devices.dart

@@ -81,6 +81,11 @@ extension DeviceApi on BackendAPI {
     await _dio.delete('/devices/$macAddress');
   }
 
+  /// Permanently deletes a device and all of its event history. This cannot be undone.
+  Future<void> deleteDevice(String macAddress) async {
+    await _dio.delete('/devices/$macAddress/permanently');
+  }
+
   Future<List<DeviceEvent>> getDeviceEvents(
     String macAddress, {
     DateTime? createdFrom,