feat(backend): add auto-launch functionality

Implement system auto-launch feature to allow CC-Switch to start
automatically on system boot, improving user convenience.

Backend Implementation:
- auto_launch.rs: New module for auto-launch management
  * Cross-platform support using auto-launch crate
  * Enable/disable auto-launch with system integration
  * Proper error handling for permission issues
  * Platform-specific implementations (macOS/Windows/Linux)

Command Layer:
- Add get_auto_launch command to check current status
- Add set_auto_launch command to toggle auto-start
- Integrate commands with settings API

Settings Integration:
- Extend Settings struct with auto_launch field
- Persist auto-launch preference in settings store
- Automatic state synchronization on app startup

Dependencies:
- Add auto-launch ^0.5.0 to Cargo.toml
- Update Cargo.lock with new dependency tree

Technical Details:
- Uses platform-specific auto-launch mechanisms:
  * macOS: Login Items via LaunchServices
  * Windows: Registry Run key
  * Linux: XDG autostart desktop files
- Handles edge cases like permission denials gracefully
- Maintains settings consistency across app restarts

This feature enables users to have CC-Switch readily available
after system boot without manual intervention, particularly useful
for users who frequently switch between API providers.

src-tauri/src/commands/settings.rs

@@ -37,3 +37,20 @@ pub async fn set_app_config_dir_override(
     crate::app_store::set_app_config_dir_to_store(&app, path.as_deref())?;
     Ok(true)
 }
+
+/// 设置开机自启
+#[tauri::command]
+pub async fn set_auto_launch(enabled: bool) -> Result<bool, String> {
+    if enabled {
+        crate::auto_launch::enable_auto_launch().map_err(|e| format!("启用开机自启失败: {e}"))?;
+    } else {
+        crate::auto_launch::disable_auto_launch().map_err(|e| format!("禁用开机自启失败: {e}"))?;
+    }
+    Ok(true)
+}
+
+/// 获取开机自启状态
+#[tauri::command]
+pub async fn get_auto_launch_status() -> Result<bool, String> {
+    crate::auto_launch::is_auto_launch_enabled().map_err(|e| format!("获取开机自启状态失败: {e}"))
+}