feat: add SPA, serve at /app/, update Dockerfile and README

- React + TanStack Router + shadcn/ui SPA under spa/
- serve spa/dist at /app/ with index.html fallback for client routing
- Dockerfile: node build stage for SPA, copy dist into runtime image
- README: document SPA, CORS_ORIGINS env var, architecture entry
- vite base set to /app/, manifest.json paths fixed

spa/.prettierignore

@@ -0,0 +1,7 @@
+node_modules/
+coverage/
+.pnpm-store/
+pnpm-lock.yaml
+package-lock.json
+pnpm-lock.yaml
+yarn.lock