feat: add MBE health check proxy endpoint

- Added /api/v1/mbe/health proxy in Studio proxy (port 8888)
- Frontend now uses proxy endpoint instead of direct mbeUrl
- Works for both localhost and external access (once proxy is deployed)
- MBE health response: {status: 'healthy', model: 'e4b', layers: 42}

src-tauri/Cargo.lock

@@ -3766,6 +3766,7 @@ dependencies = [
  "ctr 0.9.2",
  "dashmap",
  "dav-server",
+ "dirs 5.0.1",
  "ed25519-dalek 2.2.0",
  "env_logger",
  "filetime",