refactor: 迁移前端到 React 19 + Zustand + Tailwind CSS v4

- 将 vanilla TS 单文件 (app.ts 395行) 拆分为 React 组件化架构
- 引入 Zustand 管理全局状态 (连接/录音/预览/历史/toast)
- 自定义 hooks 封装 WebSocket 连接和音频录制管线
- CSS 全面 Tailwind 化，style.css 从 234 行精简到 114 行 (仅保留 tokens + keyframes)
- 新增依赖: react, react-dom, zustand, @vitejs/plugin-react
- Go 后端 embed 路径 web/dist 不变，无需改动

web/src/App.tsx

@@ -0,0 +1,33 @@
+import { HistoryList } from "./components/HistoryList";
+import { MicButton } from "./components/MicButton";
+import { PreviewBox } from "./components/PreviewBox";
+import { StatusBadge } from "./components/StatusBadge";
+import { Toast } from "./components/Toast";
+import { useRecorder } from "./hooks/useRecorder";
+import { useWebSocket } from "./hooks/useWebSocket";
+
+export function App() {
+	const { sendJSON, sendBinary } = useWebSocket();
+	const { startRecording, stopRecording } = useRecorder({
+		sendJSON,
+		sendBinary,
+	});
+
+	return (
+		<>
+			<div className="relative z-1 mx-auto flex h-full max-w-[480px] flex-col px-5 pt-[calc(16px+env(safe-area-inset-top,0px))] pb-[calc(16px+env(safe-area-inset-bottom,0px))]">
+				<header className="flex shrink-0 items-center justify-between pt-2 pb-5">
+					<h1 className="font-bold text-[22px] tracking-[-0.03em]">
+						VoicePaste
+					</h1>
+					<StatusBadge />
+				</header>
+
+				<PreviewBox />
+				<MicButton onStart={startRecording} onStop={stopRecording} />
+				<HistoryList sendJSON={sendJSON} />
+			</div>
+			<Toast />
+		</>
+	);
+}