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>

Dockerfile

@@ -1,10 +1,18 @@
 # syntax=docker/dockerfile:1
+FROM node:22-alpine AS frontend
+WORKDIR /ui
+COPY ui/package.json ui/package-lock.json ui/.npmrc ./
+RUN npm ci
+COPY ui/ .
+RUN npm run build
+
 FROM eclipse-temurin:21-jdk-alpine AS build
 WORKDIR /build
 COPY .mvn/ .mvn/
 COPY mvnw pom.xml ./
 RUN --mount=type=cache,target=/root/.m2/repository ./mvnw dependency:go-offline -B
 COPY src/ src/
+COPY --from=frontend /ui/dist/ src/main/resources/static/
 RUN --mount=type=cache,target=/root/.m2/repository ./mvnw package -DskipTests -B
 
 FROM eclipse-temurin:21-jre-alpine