fix: make file requests dialog accessible

[kristian-zendato]

• Resolves: https://github.com/nextcloud-gmbh/customer-feature-requests/issues/1590

Summary

Improve a11y of the Create file request dialog:

1. Recipient emails are a semantic list — chips render in a <ul> / <li> with an accessible name (Recipient emails).
2. Unambiguous chip delete labels — each close button is Remove email: {email} instead of a shared Remove email.
3. Form labels are programmatically linked — visible legends use <label for>, fields have stable ids, helper text is aria-describedby, and the expiration date keeps a visible linked label.
4. Upload destination is keyboard-operable — Enter/Space opens the folder picker; typing/paste still do not edit the path.
5. Focus returns to New — after File request created closes, focus goes back to the New button instead of the Files keyboard-shortcuts UI.

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

[susnux]

why the ID?