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/vite.config.ts

@@ -0,0 +1,19 @@
+import path from "path"
+import { tanstackRouter } from "@tanstack/router-plugin/vite"
+import tailwindcss from "@tailwindcss/vite"
+import react from "@vitejs/plugin-react"
+import { defineConfig } from "vite"
+
+export default defineConfig({
+  base: "/app/",
+  plugins: [
+    tanstackRouter({ target: "react", autoCodeSplitting: true }),
+    react(),
+    tailwindcss(),
+  ],
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+})