fix: show date in timestamps; fix history divider appearing immediately

- MessagePane: show date in timestamps (Yesterday/MMM D) for messages
  not from today; divider now appears at the top of history on load
  rather than waiting for a live message to create the boundary
- TUI: same date-aware timestamp formatting (Yesterday / Jan 2)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/tui/main.go

@@ -383,7 +383,7 @@ func (m model) refreshViewport() model {
 	w := m.vpContentWidth()
 	var sb strings.Builder
 	for _, e := range m.messages[room] {
-		ts := styleMsgTime.Render(e.at.Format("15:04"))
+		ts := styleMsgTime.Render(formatMsgTime(e.at))
 		var from string
 		if e.fromMe {
 			from = styleMsgMe.Render(e.from)
@@ -614,6 +614,17 @@ func filterIDs(ids []proto.PeerID, remove proto.PeerID) []proto.PeerID {
 	return out
 }
 
+func formatMsgTime(t time.Time) string {
+	now := time.Now()
+	if t.Year() == now.Year() && t.YearDay() == now.YearDay() {
+		return t.Format("15:04")
+	}
+	if t.Year() == now.Year() && t.YearDay() == now.YearDay()-1 {
+		return "Yesterday " + t.Format("15:04")
+	}
+	return t.Format("Jan 2 15:04")
+}
+
 func min(a, b int) int {
 	if a < b {
 		return a