useConversations reported unreadCount: 0 for everything, so the inbox never
showed new messages. Add a lightweight, reliable unread signal:
- useDMRead: per-conversation, per-user last-read timestamps in localStorage
(local-only; compares against the inner rumor's real created_at, not the
fuzzed gift-wrap time, so it's accurate).
- DirectMessagesPage: a conversation is unread when its latest message is from
the other person and newer than last-read — rendered as a red dot, bolder
name, brighter preview text, and a subtle highlighted card.
- ConversationPage: opening a chat marks it read up to the newest message, so
the dot clears on return (each page mounts fresh, so no stale state).
Scope: kept to the Messages page. Deliberately did NOT add a global nav badge,
which would require background-decrypting all gift wraps on every page — the
same heavy cost avoided for the crew bell.
Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>
