Tier0-App-Scaffold

Diagnose and fix managed preview startup failures for TanStack Start or Vite apps in UNS-SWE executor sandboxes. Use when the user reports "Failed to load preview", "Preview loading timed out", "dev server exited: exit status 1", `{"status":500,"unhandled":true,"message":"HTTPError"}`, Tier0 SDK SSR/noExternal issues, preview_start, preview/status, live preview, Vite dev server, port 5173, /api/health, stale npm/node processes, sandbox preview process cleanup problems, missing node_modules or uninstalled dependencies, npm install/npm ci dependency restore (e.g. uploaded projects shipped without node_modules), or frequent full page reloads / broken HMR / React Fast Refresh / Vite optimizeDeps re-optimization during development.