Render review drawer drafts like notes in PreviewItem

What does this merge request do and why?

Each pending comment in the review drawer now looks like a normal diff discussion note. Before this change, each one showed a generic icon, a link-style title, and the comment as plain text cut to 3 lines with the markdown removed. Now PreviewItem shows the file header for diff comments, a "Comment on line X" row, the author with NoteHeader and the full rendered markdown. A reply to a general thread shows a "Reply to X's thread" row.

Each pending comment also has edit and delete buttons that use the existing updateDraft and deleteDraft actions. The edit state is local to the drawer, so it does not also open the inline draft editor in the diff. Reviewers can now read and fix their pending comments before they submit the review, without a return to the diff.

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