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/i18n/locales/en.json

@@ -166,6 +166,9 @@
     "languageOptionEnglish": "English",
     "windowBehavior": "Window Behavior",
     "windowBehaviorHint": "Configure window minimize and Claude plugin integration policies.",
+    "launchOnStartup": "Launch on Startup",
+    "launchOnStartupDescription": "Automatically run CC Switch when system starts",
+    "autoLaunchFailed": "Failed to set auto-launch",
     "minimizeToTray": "Minimize to tray on close",
     "minimizeToTrayDescription": "When checked, clicking the close button will hide to system tray, otherwise the app will exit directly.",
     "enableClaudePluginIntegration": "Apply to Claude Code extension",

src/i18n/locales/zh.json

@@ -166,6 +166,9 @@
     "languageOptionEnglish": "English",
     "windowBehavior": "窗口行为",
     "windowBehaviorHint": "配置窗口最小化与 Claude 插件联动策略。",
+    "launchOnStartup": "开机自启",
+    "launchOnStartupDescription": "随系统启动自动运行 CC Switch",
+    "autoLaunchFailed": "设置开机自启失败",
     "minimizeToTray": "关闭时最小化到托盘",
     "minimizeToTrayDescription": "勾选后点击关闭按钮会隐藏到系统托盘，取消则直接退出应用。",
     "enableClaudePluginIntegration": "应用到 Claude Code 插件",