feat(web): externalize telemetry URL via environment variables

Replace the hardcoded Nginx proxy configuration with a dynamic environment variable approach. This allows the telemetry dashboard to be loaded via a public URL instead of relying on a local Tailscale proxy.

- Add `VITE_TELEMETRY_URL` build argument to Dockerfile
- Pass `VITE_TELEMETRY_URL` through GitHub Actions deployment workflow
- Remove `/telemetry/` location block from nginx.conf
- Update `TelemetryPage` to use the new environment variable and improve iframe loading state management

apps/web/nginx.conf

@@ -21,23 +21,6 @@ server {
     proxy_set_header X-Forwarded-Proto $scheme;
   }
 
-  # Proxy telemetry dashboard to orange VPS (via Tailscale)
-  location /telemetry/ {
-    proxy_pass http://100.96.248.86:8181/;
-    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 X-Forwarded-Proto $scheme;
-    proxy_connect_timeout 5s;
-    proxy_send_timeout 35s;
-    proxy_read_timeout 35s;
-    proxy_buffering on;
-    proxy_buffer_size 4k;
-    proxy_buffers 8 4k;
-    proxy_busy_buffers_size 8k;
-  }
-
   location / {
     try_files $uri $uri/ /index.html;
   }