Add self-hosted time-based sync demo

[bean1352]

A React demo that shows how to use PowerSync sync streams to dynamically control which data is synced to the client based on a date field. Issues are grouped by updated_at date, and each date is a separate stream subscription that the user can toggle on or off. Runs entirely locally with self-hosted PowerSync and Supabase.

• Self-hosted PowerSync + local Supabase setup using PowerSync CLI and supabase start
• Anonymous auth, well-known local anon key hardcoded in env template
• Supabase migrations for schema, seed data, and powersync publication
• Per-date useQuery that both subscribes to the sync stream and renders matching issues (TTL 0)

TODO: Add demo link to this guide.

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Latest commit: ec54060

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[benitav]

Just a hygiene thing, we should also add a reference to this demo in the root README https://github.com/powersync-ja/powersync-js

[Chriztiaan]

Looks good from my perspective. We cleared up the flicker issues we caught with stream changing and fixed them in the @powersync/react package.