chore(deps): resolve open Dependabot security alerts (#1191)

Bump two dependencies flagged by Dependabot ahead of the v1.14.0 release:

- mcp 1.25.0 -> 1.28.1 (3 high alerts). Transitive via content-core ->
  fastmcp; the advisories cover MCP *server* transports (missing
  Host/Origin validation on the WebSocket transport, HTTP transports
  serving session requests without verifying the authenticated
  principal, and experimental task handlers exposing other clients'
  tasks). Open Notebook does not serve MCP, so none of them are
  reachable here, but fastmcp accepts the newer release and the bump
  resolves cleanly with no other dependency churn.
- axios 1.16.0 -> 1.18.1 (medium alert). Prototype pollution allowing
  Basic auth injection through auth subfields. Direct frontend
  dependency; pulls nested agent-base/https-proxy-agent, which newer
  axios uses for proxy handling.

Verified: uv sync + content-core/fastmcp import clean, frontend
npm ci + npm run build green, npm audit reports 0 vulnerabilities.

Author: lfnovo

frontend/package-lock.json

@@ -34,7 +34,7 @@
     "@tanstack/react-query": "^5.83.0",
     "@types/react-syntax-highlighter": "^15.5.13",
     "@uiw/react-md-editor": "^4.0.8",
-    "axios": "^1.16.0",
+    "axios": "^1.18.1",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",