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/nginx.conf

@@ -1,28 +0,0 @@
-server {
-    listen 8334;
-    server_name _;
-
-    root /usr/share/nginx/html;
-    index index.html;
-
-    # Bitcoin RPC proxy to avoid CORS issues
-    location /bitcoin-rpc/ {
-        # Proxy to localhost Bitcoin RPC (using host network mode)
-        proxy_pass http://127.0.0.1:8332/;
-        proxy_http_version 1.1;
-        proxy_set_header Host $host;
-        proxy_set_header X-Real-IP $remote_addr;
-        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header Authorization "Basic __BITCOIN_RPC_AUTH__";
-        add_header Access-Control-Allow-Origin *;
-        add_header Access-Control-Allow-Methods "POST, GET, OPTIONS";
-        add_header Access-Control-Allow-Headers "Content-Type, Authorization";
-        if ($request_method = OPTIONS) {
-            return 204;
-        }
-    }
-
-    location / {
-        try_files $uri $uri/ /index.html;
-    }
-}