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>

docker/bitcoin-ui/Dockerfile

@@ -1,20 +0,0 @@
-FROM docker.io/library/nginx:alpine
-
-# Copy the static UI
-COPY index.html /usr/share/nginx/html/
-COPY tailwind.css /usr/share/nginx/html/
-
-# Create assets directories first
-RUN mkdir -p /usr/share/nginx/html/assets/img/app-icons && \
-    mkdir -p /usr/share/nginx/html/assets/img
-
-# Copy Bitcoin Knots icon and background
-COPY bitcoin-knots.webp /usr/share/nginx/html/assets/img/app-icons/
-COPY bg-network.jpg /usr/share/nginx/html/assets/img/
-
-# Copy custom Nginx config with Bitcoin RPC proxy
-COPY nginx.conf /etc/nginx/conf.d/default.conf
-
-EXPOSE 80
-
-CMD ["nginx", "-g", "daemon off;"]