Give palettes one active row, listbox semantics, and a stable top anchor

Keyboard, pointer, and a non-empty query share a single highlight; Home and
End jump between rows; Enter acts on the pre-highlighted first match. Rows
are 44px with a visible Add hint on coarse pointers, loading and empty
states announce as status, palettes anchor near the top, the search repo
suffix depends on project count, and palette spacing uses tokens.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>