Rewrite mobile app plan — per-instance architecture + Firebase + demo auth

- One app per client: single instance.dart config file change = new build
- Firebase: one project per client, service account JSON uploaded to system settings
- Auth: demo mode (OTP=123456) for testing, real SMS when provider configured
- Runtime branding: logo/colors fetched from /api/v1/app/config, not hardcoded
- Full Firebase setup checklist, notification flow diagram, deployment checklist
- New client deployment: Firebase (5min) + Flutter build (5min) + config (2min)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>