feat: enrich footer status details

[Gerrri]

Summary

• move the connection status indicator from the header into the footer area
• show separate backend and coordinator indicators with subtle colored dots
• display the dashboard version in the footer from runtime configuration
• use the production image build version automatically and fall back to dev in the normal development path
• update the App test expectations to match the new footer UI and version behavior

Notes

• Fixes Feature: Show dashboard version in footer #28

Verification

• npm test -- --run src/tests/App.test.tsx (run from frontend/)
• docker compose --profile staging up -d --build
• docker build -f Dockerfile.prod --build-arg APP_VERSION=v9.9.9-test -t labgrid-dashboard-prod-version-test .
• docker run --rm --entrypoint /bin/bash labgrid-dashboard-prod-version-test -lc '/entrypoint.sh true >/tmp/entry.log 2>&1; cat /var/www/html/env-config.js'
• curl -I http://localhost:3000