- Add missing admin.ntfy.hint translation key in all 15 languages
- Add admin ntfy server hint clarifying it is the default for users
- Expose admin_ntfy_server via PreferencesMatrix so user settings
  placeholder reflects the admin-configured default
- Add clear token button to admin ntfy panel (same pattern as user settings)
- Extract common.clear from settings.ntfyUrl.clearToken across all 15 languages