React Native (Expo) + Supabase: AsyncStorage adapter, URL polyfill, and the scheme redirect

# React Native (Expo) + Supabase: the three shims supabase-js needs

`supabase-js` was built for browsers. On React Native there is no `localStorage`, no `URL` global in older setups, and no automatic return from the OAuth browser tab. Agents that just `createClient` get an app where login works until the app restarts, then the session is gone.

## Checkable procedure

1. Pass an auth storage adapter backed by `AsyncStorage` in the client options (`auth: { storage: AsyncStorage }`). Default storage does not persist on native, so sessions vanish on restart.
2. Install and import the URL polyfill (`react-native-url-polyfill`) before creating the client. Supabase auth parses URLs during the OAuth and OTP flows, and the global is missing on Hermes/JSC.
3. Register a custom scheme in `app.json` under `expo.scheme`, and add the matching redirect URL (for example `com.supabase://**`) to the dashboard Redirect URLs.
4. After OAuth, handle the incoming link with `Linking` and let the client create the session from the URL. For magic links use `emailRedirectTo` pointing at your scheme so the tap returns to the app.
5. Set `detectSessionInUrl` appropriately for your navigation setup. With Expo Router the redirect lands in a route; make sure the client processes the URL before your auth gate decides the user is logged out.
6. Never bundle the service role key in the app. It ships inside the binary and anyone can extract it. Admin work goes through Edge Functions.

## Quick test

Sign in, fully kill the app, reopen, and confirm the session persisted. Then do an OAuth sign-in on a real device and confirm the browser returns to the app and the session is established.