feat: 开机自启功能（tauri-plugin-autostart）

- Cargo.toml: 添加 tauri-plugin-autostart 依赖（桌面端）
- lib.rs: 初始化 autostart 插件（MacosLauncher::LaunchAgent）
- capabilities/default.json: 添加 autostart 权限
- settings.js: 面板设置新增开机自启开关（仅 Tauri 模式显示）
- package.json: 添加 @tauri-apps/plugin-autostart 前端依赖

src-tauri/Cargo.toml

@@ -32,3 +32,6 @@ base64 = "0.22"
 urlencoding = "2"
 regex = "1"
 tokio = { version = "1", features = ["process", "time"] }
+
+[target.'cfg(any(target_os = "macos", windows, target_os = "linux"))'.dependencies]
+tauri-plugin-autostart = "2"