Round 17 adopted main's #8850 ComposerBanner.Attachment (mx-auto plus the
standalone drawer-inset width) without main's matching mounts, so the
stash tab's ml-auto lost to the attachment's auto right margin and the
tab centered over the composer. Column now spans its attachments like
main does, the stash tab zeroes the right margin, and the stash menu
keeps the full dock width.

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