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/public/assets/img/app-icons/thunderhub.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" fill="none">
+  <circle cx="32" cy="32" r="30" fill="#1a1a2e" stroke="#f7931a" stroke-width="2"/>
+  <path d="M36 12L22 34h10l-4 18 18-24H34l2-16z" fill="#f7931a"/>
+</svg>