Step 14 - comments

Add commenting using the comments subcollection from prompt 02.

• One collection, two views: comments without elementId form the board-wide discussion; comments with elementId form a pinned thread on that element.
• Comments side panel: board thread by default; selecting "comment" on an element (context bar action) switches the panel to that element's thread, with a back link to the board thread.
• Draw a small comment badge with a count on every element that has a thread (canvas-drawn, from a commentCounts computed map). Clicking the badge opens that thread.
• Unread indicator on the panel toggle: compare the latest comment createdAt against a locally stored commentsReadAt per board.
• Permissions per the rules: authors edit (mark editedAt) and delete their own; org members may delete any comment (moderation). Guests can comment.
• Author name + color chip on each comment; relative timestamps.

Acceptance: two windows hold a conversation on the board thread and on an element thread; badges update live; moderation delete works for a member but not for another guest.