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/README.md

@@ -0,0 +1,21 @@
+# React + TypeScript + Vite + shadcn/ui
+
+This is a template for a new Vite project with React, TypeScript, and shadcn/ui.
+
+## Adding components
+
+To add components to your app, run the following command:
+
+```bash
+npx shadcn@latest add button
+```
+
+This will place the ui components in the `src/components` directory.
+
+## Using components
+
+To use the components in your app, import them as follows:
+
+```tsx
+import { Button } from "@/components/ui/button"
+```