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/public/manifest.json

@@ -0,0 +1,12 @@
+{
+  "name": "Movies Diary",
+  "short_name": "Movies",
+  "description": "Track and review movies",
+  "start_url": "/app/",
+  "display": "standalone",
+  "background_color": "#0a0a0f",
+  "theme_color": "#0a0a0f",
+  "icons": [
+    { "src": "logo.webp", "sizes": "any", "type": "image/webp" }
+  ]
+}