[iamnbutler]

Opening a channel with new messages now shows where they start, and messages that mention you stand out.

• A New divider sits above the first message someone else wrote after you last saw the channel. It is taken as the channel opens (from the attention state that already drives the sidebar's unread marks) and bounded by the channel's last message at that moment, so messages that arrive while you watch don't move it. It also starts a fresh group, so that message's sender and time show.
• A message from someone else that @mentions your GitHub login gets an accent tint and left bar, whenever it arrives.
• Timeline (@ace/ui) takes unread (the first unseen message's id) and mentioned (ids to highlight); compile inserts a new item that's laid out like a day divider.

Unread counts across a reload already held with a persistent browser profile, as #602 suspected; no change there.

Part of #602 (the divider and the highlight; showing each grouped message's own time is not in this PR). Map: ^903, ^486.

Smoke: loopback source host with its own ACE_HOME, headless Chromium with a persistent profile. I opened #unread (two of my messages), switched away, posted two messages as rivera (host restarted with ACE_USER=rivera), one saying @iamnbutler can you look at the build?, then went back as myself. The row read "1 direct mention" before and after a reload; opening it showed the New divider between my messages and rivera's, and only the mention bubble tinted with the accent bar. Opening it again later showed no divider.

Built in Ace

No: made from Claude Code (a fix-train worker), with a headless Chromium for the smoke test. Dogfooding gap tracked in #5.