The Settings toggle label previously said 'Show CLI sessions' or 'Show
agent sessions', but the feature actually surfaces conversations from
CLI, Telegram, Discord, Slack, WeChat, and other non-WebUI channels.
- Rename i18n key: settings_label_cli_sessions → settings_label_external_sessions
- Rename i18n key: settings_desc_cli_sessions → settings_desc_external_sessions
- Update all 8 languages (en, zh, zh-TW, ru, es, de, pt, ko)
- Reorder channel examples by global adoption: Telegram, Discord, Slack
- Update HTML fallback text to match new English strings