refactor(settings): rename Dock setting to system tray (showInDock → showInTray)

- compat: map legacy showInDock to showInTray when loading settings
- ui(copy): clarify “system tray (menu bar)” vs Dock in SettingsModal
- tauri(settings): return showInTray in get_settings; adjust default fallback
- docs(comment): align comments to “system tray” terminology across code
- note: no functional change yet; tray visibility toggle remains unimplemented

src/types.ts

@@ -13,5 +13,6 @@ export interface AppConfig {
 
 // 应用设置类型（用于 SettingsModal 与 Tauri API）
 export interface Settings {
-  showInDock: boolean;
+  // 是否在系统托盘（macOS 菜单栏）显示图标
+  showInTray: boolean;
 }