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/devices/device_list_rows.dart

@@ -464,6 +464,8 @@ class _DeviceActionsMenu extends StatelessWidget {
           await confirmForgetDevice(context, device, onRefresh);
         } else if (value == 'register') {
           await showRegisterDeviceDialog(context, device, onRefresh);
+        } else if (value == 'delete') {
+          await confirmDeleteDevice(context, device, onRefresh);
         }
       },
       itemBuilder: (context) => [
@@ -474,6 +476,8 @@ class _DeviceActionsMenu extends StatelessWidget {
           const PopupMenuItem(value: 'forget', child: Text('Forget')),
         if (!device.isRegistered)
           const PopupMenuItem(value: 'register', child: Text('Register')),
+        if (!device.isRegistered)
+          const PopupMenuItem(value: 'delete', child: Text('Delete')),
       ],
     );
   }