feat(settings): add auto-launch on system startup feature

Implement auto-launch functionality with proper state synchronization
and error handling across Windows, macOS, and Linux platforms.

Key changes:
- Add auto_launch module using auto-launch crate 0.5
- Define typed errors (AutoLaunchPathError, AutoLaunchEnableError, etc.)
- Sync system state with settings.json on app startup
- Only call system API when auto-launch state actually changes
- Add UI toggle in Window Settings panel
- Add i18n support for auto-launch settings (en/zh)

Implementation details:
- Settings file (settings.json) is the single source of truth
- On startup, system state is synced to match settings.json
- Error handling uses Rust type system with proper error propagation
- Frontend optimized to avoid unnecessary system API calls

Platform support:
- Windows: HKEY_CURRENT_USER registry modification
- macOS: AppleScript-based launch item (configurable to Launch Agent)
- Linux: XDG autostart desktop file

src-tauri/src/auto_launch.rs

@@ -0,0 +1,39 @@
+use crate::error::AppError;
+use auto_launch::AutoLaunch;
+
+/// 初始化 AutoLaunch 实例
+fn get_auto_launch() -> Result<AutoLaunch, AppError> {
+    let app_name = "CC Switch";
+    let app_path = std::env::current_exe().map_err(AppError::AutoLaunchPathError)?;
+
+    let auto_launch = AutoLaunch::new(app_name, &app_path.to_string_lossy(), false, &[] as &[&str]);
+    Ok(auto_launch)
+}
+
+/// 启用开机自启
+pub fn enable_auto_launch() -> Result<(), AppError> {
+    let auto_launch = get_auto_launch()?;
+    auto_launch
+        .enable()
+        .map_err(|e| AppError::AutoLaunchEnableError(e.to_string()))?;
+    log::info!("Auto-launch enabled");
+    Ok(())
+}
+
+/// 禁用开机自启
+pub fn disable_auto_launch() -> Result<(), AppError> {
+    let auto_launch = get_auto_launch()?;
+    auto_launch
+        .disable()
+        .map_err(|e| AppError::AutoLaunchDisableError(e.to_string()))?;
+    log::info!("Auto-launch disabled");
+    Ok(())
+}
+
+/// 检查是否已启用开机自启
+pub fn is_auto_launch_enabled() -> Result<bool, AppError> {
+    let auto_launch = get_auto_launch()?;
+    auto_launch
+        .is_enabled()
+        .map_err(|e| AppError::AutoLaunchCheckError(e.to_string()))
+}