PoC:Support for Sign in with QR with MSC4388 including on a new device

[hughns]

Uses matrix-org/matrix-js-sdk#5219

This is a implementation of the 2025 version of MSC4108 to support the MSC.

Features:

• it supports generating and showing a QR on an existing Element Web
• it supports generating and showing a QR to sign in on a new Element Web

Notes for when implementing production code:

• the lifecycle changes are probably not sensible
• the crypto integration is messy
• no tests

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)