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>

.gitea/workflows/ci.yml

@@ -27,6 +27,12 @@ jobs:
           key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
           restore-keys: ${{ runner.os }}-maven-
 
+      - name: Build Frontend
+        run: |
+          cd ui
+          npm ci
+          npm run build
+
       - name: Build and Test (unit tests only)
         run: >-
           mvn clean verify -B