init: FileDrop phase1 architecture and scaffold

- Rust axum signaling server with WebSocket support
- Lit + TypeScript frontend with Vite
- Redis session storage with TTL
- WebRTC transport and crypto client stubs
- Phase1 architecture plan in plans/
- Deploy directory structure prepared

README.md

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+# FileDrop
+
+安全无痕文件传输 Web App
+
+## 特性
+
+- 免费、无注册、无历史记录
+- 优先局域网，WebRTC 直连
+- 浏览器端端到端加密
+- 传完即销毁
+
+## 技术栈
+
+- 前端：Lit + TypeScript + Vite
+- 后端：Rust + axum
+- 存储：Redis
+- NAT 穿透：coturn
+
+## 开发
+
+### 后端
+
+```bash
+cd server
+cargo run
+```
+
+### 前端
+
+```bash
+cd web
+npm install
+npm run dev
+```
+
+## 部署
+
+见 `deploy/` 目录