fix(desktop): 修复桌面端 API 连接问题

- API 客户端使用完整后端 URL (localhost:3000)
- 添加 tauri-plugin-http 支持外部 HTTP 请求
- 配置 CSP 允许连接 localhost
- 同步 useChat hook 修复 WebSocket 错误处理

packages/desktop/src-tauri/Cargo.toml

@@ -18,6 +18,7 @@ tauri = { version = "2", features = ["tray-icon"] }
 tauri-plugin-shell = "2"
 tauri-plugin-fs = "2"
 tauri-plugin-dialog = "2"
+tauri-plugin-http = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["full"] }