feat: add SPA controller, Traefik route, CI frontend build, and HOWTO update

- SpaController catch-all forwards non-API routes to index.html
- Traefik SPA route at priority=1 catches all unmatched paths
- CI pipeline builds frontend before Maven
- Dockerfile adds multi-stage frontend build
- HOWTO.md documents frontend development workflow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -79,6 +79,9 @@ services:
       - traefik.http.services.api.loadbalancer.server.port=8080
       - traefik.http.routers.forwardauth.rule=Path(`/auth/verify`)
       - traefik.http.services.forwardauth.loadbalancer.server.port=8080
+      - traefik.http.routers.spa.rule=PathPrefix(`/`)
+      - traefik.http.routers.spa.priority=1
+      - traefik.http.services.spa.loadbalancer.server.port=8080
     networks:
       - cameleer