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/lib/tauri-api.ts

@@ -201,7 +201,7 @@ export const tauriAPI = {
       return await invoke("get_settings");
     } catch (error) {
       console.error("获取设置失败:", error);
-      return { showInDock: true };
+      return { showInTray: true };
     }
   },