fix: remove qrCodeUri

[horsefacts]

Motivation

The useCreateChannel and useSignIn hooks convert the auth relay connection URL to a QR code and return it encoded as a data URI. However, the underlying library uses the Canvas API and is incompatible with React Native.

AuthKit components don't actually use this internally, instead we use the unencoded url to create a QR code SVG. Although returning this value might be convenient, it probably doesn't belong at the hook level: clients should use a QR code library of their choice to render the url as a scannable code (or use the QRCode component included in AuthKit).

Change Summary

Remove qrCodeUri from the return value of useCreateChannel and useSignIn.
Export QRCode component.

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