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>

api/app.py

@@ -629,6 +629,13 @@ def get_pending_config():
     })
 
 
+@app.route('/api/config/pending', methods=['DELETE'])
+def cancel_pending_config():
+    """Discard pending configuration changes without restarting any containers."""
+    _clear_pending_restart()
+    return jsonify({'message': 'Pending changes discarded'})
+
+
 @app.route('/api/config/apply', methods=['POST'])
 def apply_pending_config():
     """Apply pending configuration by restarting containers via docker compose up -d."""