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/ui/dialog_preferences.go

@@ -13,8 +13,7 @@ import (
 )
 
 // preferencesDialog edits the persistent UI prefs: UI language, theme,
-// and auto-lock timeout. Minimize-to-tray lives in its own bucket and
-// will appear here later.
+// auto-lock timeout, and minimize-to-tray behaviour.
 type preferencesDialog struct {
 	lang  string
 	theme themeMode
@@ -35,6 +34,10 @@ type preferencesDialog struct {
 	// input on that, the store does.
 	autoLockEd widget.Editor
 
+	// Minimize-to-tray: when on, the X button hides the window to the
+	// system tray instead of exiting. Tray icon stays visible either way.
+	minToTray widget.Bool
+
 	okBt     widget.Clickable
 	cancelBt widget.Clickable
 }
@@ -46,6 +49,7 @@ type preferencesResult struct {
 	language        string
 	theme           themeMode
 	autoLockMinutes int
+	minimizeToTray  bool
 }
 
 // supportedLanguages is the closed set the prefs dialog exposes. Keep
@@ -59,7 +63,7 @@ var supportedLanguages = []struct {
 	{"de", "Deutsch"},
 }
 
-func newPreferencesDialog(currentLang string, currentTheme themeMode, currentAutoLock int) *preferencesDialog {
+func newPreferencesDialog(currentLang string, currentTheme themeMode, currentAutoLock int, currentMinToTray bool) *preferencesDialog {
 	if currentLang == "" {
 		currentLang = "en"
 	}
@@ -69,6 +73,7 @@ func newPreferencesDialog(currentLang string, currentTheme themeMode, currentAut
 	d := &preferencesDialog{lang: currentLang, theme: currentTheme}
 	d.autoLockEd.SingleLine = true
 	d.autoLockEd.SetText(strconv.Itoa(currentAutoLock))
+	d.minToTray.Value = currentMinToTray
 	return d
 }
 
@@ -88,7 +93,12 @@ func (d *preferencesDialog) Layout(
 		if err != nil || mins < 0 {
 			mins = 0
 		}
-		onDone(preferencesResult{language: d.lang, theme: d.theme, autoLockMinutes: mins})
+		onDone(preferencesResult{
+			language:        d.lang,
+			theme:           d.theme,
+			autoLockMinutes: mins,
+			minimizeToTray:  d.minToTray.Value,
+		})
 		return layout.Dimensions{Size: gtx.Constraints.Max}
 	}
 
@@ -150,6 +160,10 @@ func (d *preferencesDialog) Layout(
 			layout.Rigid(labeledEditor(th, i18n.T("label_auto_lock_minutes"), &d.autoLockEd, "")),
 			layout.Rigid(layout.Spacer{Height: unit.Dp(2)}.Layout),
 			layout.Rigid(material.Caption(th, i18n.T("hint_auto_lock_disabled")).Layout),
+			layout.Rigid(layout.Spacer{Height: unit.Dp(12)}.Layout),
+			layout.Rigid(material.CheckBox(th, &d.minToTray, i18n.T("label_minimize_to_tray")).Layout),
+			layout.Rigid(layout.Spacer{Height: unit.Dp(2)}.Layout),
+			layout.Rigid(material.Caption(th, i18n.T("hint_minimize_to_tray")).Layout),
 		)
 	}