Cross-Platform Real-Time Location Tracker -- 2

I need a clean, lightweight location-tracking solution that works seamlessly on iOS, Android, and a simple web dashboard. The entire focus is on accurate real-time updates for individual users—think of a person who wants to see their own live position on multiple devices or securely share it with a friend for a limited time.

Core expectations
• A single code-base (React Native, Flutter, or comparable) for the mobile apps plus a small web front-end.
• Live location refresh at practical intervals without draining the battery.
• Secure sign-in (email, social, or phone OTP—open to your recommendation) with end-to-end encryption for any shared links.
• A lightweight backend (Firebase, Supabase, or Node/Express with WebSockets) that can scale modestly.
• Clear documentation so I can redeploy, change API keys, and rebrand icons if needed.

Nice-to-have (not mandatory, quote separately if significant): geofencing alerts or a short location-history log that auto-deletes after X hours.

Acceptance criteria
1. I install TestFlight / Play Store testing builds and see my dot move in near real-time on mobile and web.
2. Lat/long accuracy within 15 m in open sky, update latency under 5 s on 4G.
3. Source code handed over in a Git repo with a brief README covering build steps, environment variables, and any paid service limits.
4. Post-handover support for minor bug fixes during the first week of live use.

If you have shipped similar individual-centric trackers or mapping tools, please share a link or short demo video in your proposal.