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/types.ts

@@ -101,6 +101,8 @@ export interface Settings {
   geminiConfigDir?: string;
   // 首选语言（可选，默认中文）
   language?: "en" | "zh";
+  // 是否开机自启
+  launchOnStartup?: boolean;
   // Claude 自定义端点列表
   customEndpointsClaude?: Record<string, CustomEndpoint>;
   // Codex 自定义端点列表