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/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(),