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/test/api/devices_api_test.dart

@@ -149,6 +149,15 @@ void main() {
     await BackendAPI.instance.forgetDevice('aa:bb:cc:dd:ee:ff');
   });
 
+  test('deleteDevice DELETEs the permanently path', () async {
+    adapter.onDelete(
+      '/devices/aa:bb:cc:dd:ee:ff/permanently',
+      (server) => server.reply(200, null),
+    );
+
+    await BackendAPI.instance.deleteDevice('aa:bb:cc:dd:ee:ff');
+  });
+
   test('getDeviceEvents decodes a list of events', () async {
     adapter.onGet(
       '/devices/aa:bb:cc:dd:ee:ff/events',