feat(ui): 系统托盘 + 关闭最小化到托盘

- 原生 win32 实现的 TrayManager：Shell_NotifyIcon + 私有 message-only
  窗口承载 popup 菜单，专用 OS 线程跑 GetMessage 泵，避免引入 cgo 依赖。
- WindowSubclass 通过 SetWindowLongPtrW 子类化 Gio 主窗口，拦截
  WM_CLOSE：minimize_to_tray=true 时吞掉消息只隐藏窗口；exit 路径下
  放行让 Gio 正常关闭事件循环。
- Preferences 新增"最小化到托盘"复选框，切换后立即重装/卸载钩子，
  不需要重启程序。
- 托盘菜单提供 Show / Hide / Exit；左键单击图标等同 Show；右键弹菜单。
- 非 Windows 平台所有相关 API 退化为 ErrUnsupported / no-op stub。

internal/i18n/locales/en.toml

@@ -450,3 +450,20 @@ other = "Auto-lock after (minutes)"
 
 [hint_auto_lock_disabled]
 other = "Set to 0 to disable. Only takes effect when the configuration is encrypted."
+
+# --- System tray ---
+
+[tray_show]
+other = "Show WinAuth"
+
+[tray_hide]
+other = "Hide WinAuth"
+
+[tray_exit]
+other = "Exit"
+
+[label_minimize_to_tray]
+other = "Minimize to tray when closing the window"
+
+[hint_minimize_to_tray]
+other = "Closing the window hides it to the system tray instead of exiting."