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/.prettierrc

@@ -0,0 +1,11 @@
+{
+  "endOfLine": "lf",
+  "semi": false,
+  "singleQuote": false,
+  "tabWidth": 2,
+  "trailingComma": "es5",
+  "printWidth": 80,
+  "plugins": ["prettier-plugin-tailwindcss"],
+  "tailwindStylesheet": "src/index.css",
+  "tailwindFunctions": ["cn", "cva"]
+}