fix: BUG-33 CPU threshold, TASK-27 tab icons, TASK-36 iframe errors

- BUG-33: CPU load alert threshold increased from 2x to 4x core count
  (8→16 on 4-core machine) to reduce false alerts during container ops
- TASK-27: Launch buttons for new-tab apps now show external link icon
  (BTCPay, Grafana, PhotoPrism, Portainer, OnlyOffice, etc.)
- TASK-36: Iframe error screen now distinguishes between X-Frame-Options
  blocked vs container not reachable, with appropriate messaging

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/package.json

@@ -19,7 +19,6 @@
     "build:production": "NODE_ENV=production vue-tsc -b && vite build --mode production",
     "preview": "vite preview",
     "type-check": "vue-tsc --noEmit",
-    "prebuild": "cp ../../loop-start.mp3 public/assets/audio/ 2>/dev/null || true",
     "generate-pwa-icons": "pwa-assets-generator --preset minimal-2023 public/assets/icon/favico-black.svg && cp public/assets/icon/favicon.ico public/favicon.ico",
     "generate-welcome-speech": "node scripts/generate-welcome-speech.js"
   },