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>

src/main/java/net/siegeln/cameleer/saas/config/SpaController.java

@@ -0,0 +1,13 @@
+package net.siegeln.cameleer.saas.config;
+
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.GetMapping;
+
+@Controller
+public class SpaController {
+
+    @GetMapping(value = {"/", "/login", "/callback", "/environments/**", "/license"})
+    public String spa() {
+        return "forward:/index.html";
+    }
+}