ci(ios): add --groups for external TestFlight distribution

Fastlane requires a group name when distributing externally.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Claude

.github/workflows/ios-release.yml

@@ -165,6 +165,7 @@ jobs:
             --api_key_path "$API_KEY_JSON" \
             --app_platform ios \
             --distribute_external true \
+            --groups "External Testers" \
             --notify_external_testers true \
             --skip_waiting_for_build_processing false \
             --changelog "Build $MARKETING_VERSION ($CURRENT_PROJECT_VERSION)"