Step 8 - card comments

Add comments to cards. Create a reusable comment thread component backed by a comments collection (author uid + name, markdown text, createdAt, edit and delete for the author, soft delete). The thread for a card is created LAZILY: no comment documents or thread state exist until the first comment is posted. Embed the thread at the bottom of the task panel.

When a comment is posted, also update the task's lastCommentAt and lastCommentBy and bump the board's lastActivityAt. The card face shows a speech-bubble chip when a card has comments. Keep the thread component generic (it takes a parent ref), because request-form pages will reuse it in a later phase.