fix(core): Hide current-app label at narrower viewports

[pringelmann]

• Resolves: App Navigation Redesign #59888

Summary

Preview url: https://nc-preview.tail22c02b.ts.net/

Long section names like "Appearance and accessibility" in admin areas could push the current-app trigger past the header. The existing max-width cap never fires in practice; realistic labels stay well under it. Hide the label by default and restore it at ≥ 1400px where the layout has room, with a tighter truncation cap as a safety net for long localized names.

In future we should consider making this robust by adding all header items to the same flex container.

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)

[pringelmann]

/backport to stable34

[susnux]

Any reason for that breaking point? Its different from the normal ones we have (e.g. 1024)

[pringelmann]

1400 is a stopgap. The deeper issue is that the centered search in #60665 is absolutely positioned, so CSS can't shrink the trigger label against available space. The choices are a static cap (this), a ResizeObserver, or restructuring the header so the trigger and search are flex siblings and the label can shrink/truncate naturally.

This happens to be the viewport where, at the search's max width, the abs search bar shouldn't visually collide with realistic trigger widths. (e.g. Appearance and accessibility)

The flex restructure is the proper fix but this work needs to be merged asap. I have made a note to follow up on this post-release (along with all the other deferred waffle menu improvements)