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/components/settings/WindowSettings.tsx

@@ -19,6 +19,13 @@ export function WindowSettings({ settings, onChange }: WindowSettingsProps) {
         </p>
       </header>
 
+      <ToggleRow
+        title={t("settings.launchOnStartup")}
+        description={t("settings.launchOnStartupDescription")}
+        checked={!!settings.launchOnStartup}
+        onCheckedChange={(value) => onChange({ launchOnStartup: value })}
+      />
+
       <ToggleRow
         title={t("settings.minimizeToTray")}
         description={t("settings.minimizeToTrayDescription")}