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/index.ts

@@ -0,0 +1,16 @@
+import i18next from "i18next"
+import { initReactI18next } from "react-i18next"
+import en from "./locales/en/translation.json"
+
+i18next.use(initReactI18next).init({
+  lng: "en",
+  fallbackLng: "en",
+  resources: {
+    en: { translation: en },
+  },
+  interpolation: {
+    escapeValue: false,
+  },
+})
+
+export default i18next

spa/src/i18n/locales/en/translation.json

@@ -0,0 +1,5 @@
+{
+  "app": {
+    "title": "Movies Diary"
+  }
+}