replace Start prerender with Vite SPA

vite.config.ts

@@ -1,18 +1,15 @@
 import { defineConfig } from 'vite'
 import { devtools } from '@tanstack/devtools-vite'
-
-import { tanstackStart } from '@tanstack/react-start/plugin/vite'
+import { tanstackRouter } from '@tanstack/router-plugin/vite'
 
 import viteReact from '@vitejs/plugin-react'
 import tailwindcss from '@tailwindcss/vite'
 
-// SPA mode: auth is Bearer-token client-side (localStorage), so route guards
-// and data fetching must not run during SSR.
 const config = defineConfig({
   resolve: { tsconfigPaths: true },
   plugins: [
     devtools(),
-    tanstackStart({ spa: { enabled: true } }),
+    tanstackRouter({ target: 'react', autoCodeSplitting: true }),
     tailwindcss(),
     viteReact(),
   ],