chore: update dependencies and configuration for Vite and Vitest

- Bump versions of @vitejs/plugin-react, @vitest/coverage-istanbul, @vitest/coverage-v8, and @vitest/ui to their beta releases.
- Upgrade Vite and Vitest to their respective beta versions.
- Adjust Vite configuration to disable code splitting for improved React initialization stability.

frontend/vite.config.ts

@@ -16,14 +16,13 @@ export default defineConfig({
   build: {
     outDir: 'dist',
     sourcemap: true,
-    // TEMPORARY: Disable code splitting to diagnose React initialization issue
-    // If this works, the problem is module loading order in async chunks
     chunkSizeWarningLimit: 2000,
-    rollupOptions: {
+    rolldownOptions: {
       output: {
-        // Disable code splitting - bundle everything into one file
-        manualChunks: undefined,
-        inlineDynamicImports: true
+        // Disable code splitting — single bundle for React init stability
+        // codeSplitting: false is the Rolldown-native approach
+        // (inlineDynamicImports is deprecated in Rolldown)
+        codeSplitting: false
       }
     }
   }