feat: pending-restart banner + Apply button for config changes

When ip_range changes, a persistent amber banner appears at the top of
every page showing what changed and a "Apply Now" button.  Clicking it
shows a confirmation modal ("containers will restart briefly"), then
calls POST /api/config/apply which runs docker compose up -d from inside
the API container — no manual make start needed.

Backend:
- _set_pending_restart() / _clear_pending_restart() helpers track state
  in config_manager so it survives page refresh
- GET /api/config/pending returns { needs_restart, changed_at, changes }
- POST /api/config/apply runs docker compose up -d via the mounted
  docker.sock, using the project working_dir label to resolve host paths
- docker-compose.yml mounts docker-compose.yml itself read-only into
  the API container so docker compose can read it from inside

Frontend (App.jsx):
- Polls /api/config/pending every 5 s alongside the health check
- PendingRestartBanner component with confirmation modal
- Optimistically clears banner on Apply click; API and containers
  restart in the background

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -202,6 +202,7 @@ services:
       - ./data/logs:/app/api/data/logs
       - /var/run/docker.sock:/var/run/docker.sock
       - ./.env:/app/.env.compose
+      - ./docker-compose.yml:/app/docker-compose.yml:ro
     pid: host
     restart: unless-stopped
     networks: