Adds a visible × dismiss control to MentionDropdown (shared by the fresh-@ and
re-edit paths) and, for the re-edit path which has no Tiptap suggestion plugin
to forward keys, focuses the search input on open and handles its own keyboard:
Escape dismisses (AC-4), Arrow/Enter reuse the exported selection logic so the
dropdown is navigable on its own (AC-9 parity with the fresh-@ dropdown).
Both close paths (Escape + ×) leave the mention node attrs + text byte-identical
(AC-4) — close() never touches the document. Controller wires ondismiss=close
(+refocus editor) and focusOnMount only for the re-edit open.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>