Main's #6636 workspace file drop (drag anywhere over the chat column to
attach, with the dashed overlay) split across ChatView and the
composer. The round-10 rebase restored ChatView from the pre-#6636
backup wholesale and only the composer half was ever re-applied, so
the drop target, overlay, and drag-state plumbing vanished while
addDroppedFiles sat unused on the composer handle. Re-applies main's
ChatView half verbatim; the shared workspaceFileDrop module was
already identical to main.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>