Make magic link logins work better with in-app browsers

There's been a few articles recently about magic links + passkeys:
- https://recyclebin.zip/posts/annoyinglinks/
- https://rmondello.com/2025/01/02/magic-links-and-passkeys/

We should consider some improvements to homeauth to make them work better with in-app browsers on mobile devices. Some thoughts:
- Include a fallback one-time-use code that a user can enter if the link doesn't work
- Have the magic link log in the original session (i.e. where the link was generated) instead of logging in where the link was generated
- As a downside, this means we need some sort of "wait" handler in the original login flow
- Clicking on a magic link should actually open a page with a `` which self-submits; this allows things to "open" the magic link without invalidating it.
- If a link has expired, tell the user and offer a one-click "click here to send a new one" button, instead of just erroring.

Some relevant issues from other projects:
- https://github.com/FusionAuth/fusionauth-issues/issues/629
- https://github.com/FusionAuth/fusionauth-issues/issues/2443
- https://github.com/opencollective/opencollective-frontend/pull/5476