feat(core): allow switching 2FA method from the challenge page

[skjnldsv]

• Resolves: #

Summary

Once a 2FA method is picked on the selection page, the challenge page only offers "Use backup code" and "Cancel login". If I pick Nextcloud Notification but have no logged-in device at hand, the only way to use TOTP instead is "Cancel login", which logs out and means typing the password again.

This adds a "Use another method" link to the challenge page that goes back to /login/selectchallenge, keeping redirect_url. The pending 2FA session is untouched since selectChallenge is a plain GET, so nothing changes on the session side.

The link only shows when there is another regular provider than the current one. With only TOTP + backup codes, the existing "Use backup code" link already covers it, so it stays hidden. On the backup codes page it shows as soon as a regular provider exists.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[skjnldsv]

Created it from the wrong origin...