feat(ui): 导出/备份 — otpauth URI 列表 + 加密备份导出/恢复

- qr/export.go: EntryToOtpAuth + OtpAuth.URI，将 config.Entry 反向
  转换为标准 otpauth:// URI（Battle.Net/Steam 标记为 partial）
- dialog_export.go: Tab 切换对话框，otpauth URI 列表可复制 +
  加密备份文件导出（独立密码，0o600 权限）
- dialog_restore_backup.go: 从 .winauth.bak 恢复，密码验证后合并
- settings_menu: 新增 Export / Restore Backup 菜单项
- i18n: en/zh-CN/de 三语翻译

internal/ui/app.go

@@ -100,6 +100,8 @@ type appState struct {
 	changePwDlg  *changePasswordDialog
 	welcomeDlg   *welcomeDialog
 	importDialog *importLegacyDialog
+	exportDlg    *exportDialog
+	restoreDlg   *restoreBackupDialog
 	hotkeyDialog *hotkeyDialog
 	hotkeyTarget *entry
 	tradesDialog *steamTradesDialog
@@ -556,6 +558,10 @@ func drawFrame(gtx layout.Context, th *material.Theme, st *appState, w *app.Wind
 				st.changePwDlg = newChangePasswordDialog()
 			case settingsActionImportLegacy:
 				st.importDialog = newImportLegacyDialog()
+			case settingsActionExport:
+				st.exportDlg = newExportDialog(st.snapshotEntries())
+			case settingsActionRestoreBackup:
+				st.restoreDlg = newRestoreBackupDialog()
 			case settingsActionPreferences:
 				lang, theme, autoLock, minToTray := st.store.Preferences()
 				st.prefsDialog = newPreferencesDialog(lang, normalizeThemeMode(theme), autoLock, minToTray)
@@ -606,6 +612,25 @@ func drawFrame(gtx layout.Context, th *material.Theme, st *appState, w *app.Wind
 		})
 	}
 
+	if st.exportDlg != nil {
+		return st.exportDlg.Layout(gtx, th, func(r exportResult) {
+			st.exportDlg = nil
+			w.Invalidate()
+		})
+	}
+
+	if st.restoreDlg != nil {
+		return st.restoreDlg.Layout(gtx, th, func(r restoreBackupResult) {
+			if !r.cancel && r.cfg != nil {
+				st.mergeImportedConfig(r.cfg)
+				st.store.Push()
+				st.toast.Show(i18n.T("msg_restore_done"), w)
+			}
+			st.restoreDlg = nil
+			w.Invalidate()
+		})
+	}
+
 	if st.vendorMenu != nil {
 		if v, closed := st.vendorMenu.Pick(gtx); closed {
 			st.vendorMenu = nil