feat(pending-banner): add Discard button to cancel pending restart without applying

- DELETE /api/config/pending endpoint calls _clear_pending_restart()
- cellAPI.cancelPending() calls the new endpoint
- PendingRestartBanner shows a "Discard" button alongside "Apply Now";
  clicking it drops the pending state without restarting any containers

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

webui/src/services/api.js

@@ -44,6 +44,7 @@ export const cellAPI = {
   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'),
+  cancelPending: () => api.delete('/api/config/pending'),
   applyPending: () => api.post('/api/config/apply'),
 };