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/src/i18n.ts

@@ -0,0 +1,12 @@
+import i18n from "i18next"
+import { initReactI18next } from "react-i18next"
+import en from "./locales/en.json"
+
+i18n.use(initReactI18next).init({
+  resources: { en: { translation: en } },
+  lng: "en",
+  fallbackLng: "en",
+  interpolation: { escapeValue: false },
+})
+
+export default i18n