feat: show recent people on contact icon

[kristian-zendato]

• Resolves: Make People menu more inviting #60304

Summary

Makes the People (contacts) header menu more inviting by showing a stack of up to 3 overlapping avatars of recent contacts on the menu trigger, instead of only the contacts icon.

• Adds GET /contactsmenu/preview-avatars which returns the first 3 entries from the same unfiltered contacts list as the menu (Manager::getEntries with an empty filter), with optional team filtering matching the existing index endpoint.
• Frontend loads those contacts on mount and shows an NcAvatar stack when at least 2 are available; otherwise keeps the existing contacts icon.
• Widens the header trigger (width: fit-content) so the stack does not overlap notifications/profile and remains fully clickable.
• Adds PHP and Vue unit tests for the new endpoint and trigger behavior.

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