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>

webui/src/services/api.js

@@ -43,6 +43,8 @@ export const cellAPI = {
   deleteBackup: (id) => api.delete(`/api/config/backups/${id}`),
   exportConfig: (format = 'json') => api.get('/api/config/export', { params: { format } }),
   importConfig: (config, format = 'json') => api.post('/api/config/import', { config, format }),
+  getPending: () => api.get('/api/config/pending'),
+  applyPending: () => api.post('/api/config/apply'),
 };
 
 // Network Services API