fix(files_external): show that an unrestricted storage applies to all

[pringelmann]

Summary

An admin-configured external storage with an empty "Restricted to" field is available to every account. The problem is that nothing in the UI explicitly says so. The old settings had an "All people" checkbox, ticked by default. The new rewrite dropped it for a bare user/group picker, so an unrestricted mount only appears as an empty input in the form.

Two UI additions:

• The storage table shows an "All accounts" chip in the "Restricted to" column when a storage has no applicable users or groups.
• The Add/Edit dialog shows a warning under the picker while it is empty.

Before	After

This is deliberately just the hint. No confirmation step on save, that is a separate UX decision.

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 (tests, reviewed)

[pringelmann]

/backport to stable35

[pringelmann]

/backport to stable34

[pringelmann]

/backport to stable33