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>

.gitea/workflows/nightly.yml

@@ -79,6 +79,7 @@ jobs:
           IMPORT_HOST_DIR=/srv/familienarchiv-staging/import
           POSTGRES_USER=archiv
           SENTRY_DSN=${{ secrets.SENTRY_DSN }}
+          VITE_SENTRY_DSN=${{ secrets.VITE_SENTRY_DSN }}
           EOF
 
       - name: Verify backend /import:ro mount is wired