feat(core): header-anchored combobox shell for unified search

[pringelmann]

• Resolves: Combobox shell: header input + anchored results popover #61651

Summary

Turns the header search into the a combobox shell. The header field is now a real combobox <input>: you type into directly, and results show in a popover anchored under it instead of a centred NcDialog. Mobile collapses to a button that opens a full-height panel with an NcTextField inside.

The popover opens on the first character and closes when cleared (Ctrl+F opens it empty). Focusing the empty field doesn't open the popover.

Not in this PR: the search orchestration bugs (result flashing, no request cancellation, "load more") go to subtask 5, and the full combobox keyboard/a11y model goes to subtask 6. The shell just wires the input to the existing search results.

Why custom, not NC components: #header is its own stacking context and NcModal teleports to body, so it can't sit below the in-header input while dimming the page (and it steals focus off the input). The input is a plain <input> because NcTextField assumes a light form background + floating label that clash with the themed header. focus-trap is already a dep and the panel reuses NcModal's exact styling so it still looks like a standard dialog. Mobile stays a plain searchbox on purpose.

TODO

• Before/after screenshots (desktop + mobile)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests are included
• Screenshots before/after for front-end changes
• Documentation has been updated or is not required
• Labels added where applicable
• Milestone added for target branch/version

AI

• The content of this PR was partly generated using AI (mainly tests and fiddly CSS animation work)