fix(app): importa useRef — corrige tela branca do gate progressivo

O gate de comunicação interna usava useRef sem importá-lo, quebrando o render
inteiro do App (ReferenceError → tela branca).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/App.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useRef } from 'react';
 import { HybridBackend } from './services/backend.ts';
 
 import { Sidebar } from './components/Sidebar.tsx';