Fix Docker dashboard embedding

[biancarosa]

Summary

• Build the Next static dashboard inside the Docker image and embed it into the Go binary before compiling with embed_dashboard.
• Serve embedded dashboard API calls through same-origin /api/proxy and /api/admin/* routes, with NETKIT_DASHBOARD_BASE_PATH / --dashboard-base-path support for path-based reverse proxies such as /netkit.
• Add Docker dashboard validation so fallback HTML fails the image build and release flow.
• Remove the dashboard's build-time Google font dependency so static export is reproducible without external font fetches.

Root Cause

The published Docker image could run a binary that served the fallback dashboard, and the frontend assumed browser-visible localhost:8080/8081 API URLs. That broke embedded Docker deployments and path-based reverse proxy setups.

Validation

• go test ./...
• cd dashboard && npm run build:static
• make test-docker-dashboard
• NETKIT_DASHBOARD_BASE_PATH=/netkit NETKIT_DOCKER_IMAGE=netkit:dashboard-validation-netkit NETKIT_DOCKER_DASHBOARD_PORT=13001 NETKIT_DOCKER_PROXY_PORT=18082 NETKIT_DOCKER_ADMIN_PORT=18083 make test-docker-dashboard