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/settings.rs

@@ -49,6 +49,9 @@ pub struct AppSettings {
     pub gemini_config_dir: Option<String>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub language: Option<String>,
+    /// 是否开机自启
+    #[serde(default)]
+    pub launch_on_startup: bool,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub security: Option<SecuritySettings>,
     /// Claude 自定义端点列表
@@ -77,6 +80,7 @@ impl Default for AppSettings {
             codex_config_dir: None,
             gemini_config_dir: None,
             language: None,
+            launch_on_startup: false,
             security: None,
             custom_endpoints_claude: HashMap::new(),
             custom_endpoints_codex: HashMap::new(),