Add doc/notification-debug-panel.md as the canonical panel reference.
Update ngrok guides, README, and analysis doc to describe independent
Backend URL, Test Mode, and Skip JWT Authentication settings, including
recommended configs for hosted test servers vs local ngrok.
Add §6 coverage for the full backend→FCM→refresh pipeline, expected
Logcat lines, and troubleshooting that distinguishes backend success
from end-to-end delivery; cross-link checklist, workflow, and §14.