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/i18n/locales/en.toml

@@ -511,3 +511,67 @@ other = "Change icon..."
 
 [dialog_icon_picker_title]
 other = "Choose icon"
+
+# --- First-run welcome dialog ---
+
+[dialog_welcome_title]
+other = "Welcome to WinAuth"
+
+[msg_welcome_intro]
+other = "Choose whether to encrypt the list of authenticators stored on disk. Encryption protects the file with a password of your choice. You can change this later in Settings."
+
+[btn_welcome_enable_password]
+other = "Enable password protection"
+
+[btn_welcome_skip]
+other = "Skip for now"
+
+# --- Export / backup ---
+
+[menu_export]
+other = "Export..."
+
+[menu_restore_backup]
+other = "Restore backup..."
+
+[dialog_export_title]
+other = "Export authenticators"
+
+[export_tab_otpauth]
+other = "otpauth:// URIs"
+
+[export_tab_backup]
+other = "Encrypted backup"
+
+[export_otpauth_intro]
+other = "Below are standard otpauth:// URIs for each entry. You can import them into other authenticator apps (Google Authenticator, Authy, etc.)."
+
+[export_partial_warning]
+other = "partial export — vendor-specific fields omitted"
+
+[export_backup_intro]
+other = "Write an encrypted backup file that can be restored on any machine. Choose a strong password — it is NOT linked to your current WinAuth password."
+
+[label_backup_path]
+other = "Backup file path"
+
+[btn_export]
+other = "Export"
+
+[msg_empty_backup_path]
+other = "Please enter a backup file path."
+
+[msg_export_done]
+other = "Backup exported successfully."
+
+[msg_export_failed]
+other = "Export failed: %s"
+
+[dialog_restore_backup_title]
+other = "Restore from backup"
+
+[restore_backup_intro]
+other = "Select an encrypted backup file (.winauth.bak) and enter the password that was set when it was created. Entries will be appended to your current list."
+
+[msg_restore_done]
+other = "Backup restored."