fix(deploy): wire VITE_SENTRY_DSN as Docker build arg for frontend GlitchTip (#645)

VITE_SENTRY_DSN is a Vite build-time variable baked into the JS bundle.
Without an ARG/ENV in the Dockerfile build stage and a build.args entry in
docker-compose.prod.yml, the SDK initialised with enabled=false regardless
of the Gitea secret value.

- frontend/Dockerfile: add ARG VITE_SENTRY_DSN + ENV before npm run build
- docker-compose.prod.yml: add build.args.VITE_SENTRY_DSN with empty fallback
- nightly.yml: write VITE_SENTRY_DSN secret into .env.staging

Requires Gitea secret VITE_SENTRY_DSN to be set to the GlitchTip project #1 DSN.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -268,6 +268,10 @@ services:
     build:
       context: ./frontend
       target: production
+      args:
+        # Vite build-time variable — baked into the JS bundle at build time.
+        # Empty default so deploys succeed before the secret is configured.
+        VITE_SENTRY_DSN: ${VITE_SENTRY_DSN:-}
     restart: unless-stopped
     depends_on:
       backend: