Sync streams demo

[simolus3]

This adds a copy of the react-supabase-todolist demo showcasing sync streams. The intention is that we could replace the original demo with this fork once sync streams are stable.

The main difference between the new demo and the original one is htat only lists themselves are always synced. The content of each individual list needs to be subscribed to, which happens by subscribing to a sync stream while the details page for that stream is open.

Tiny demo: Data is synced when the list is first opened, note that subscriptions are collected and synced across tabs:

Bildschirmaufnahme.2025-09-18.um.18.08.24.mov

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