fix(beads): add CreatedAt to group/channel creation, check channel status

- Add CreatedAt timestamp to CreateGroupBead() in beads_group.go
- Add CreatedAt timestamp to CreateChannelBead() in beads_channel.go
- Check channel status before sending in router.go sendToChannel()
  - Reject sends to closed channels with appropriate error message

Closes: gt-yibjdm, gt-bv2f97

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/router.go

@@ -829,6 +829,9 @@ func (r *Router) sendToChannel(msg *Message) error {
 	if fields == nil {
 		return fmt.Errorf("channel not found: %s", channelName)
 	}
+	if fields.Status == beads.ChannelStatusClosed {
+		return fmt.Errorf("channel %s is closed", channelName)
+	}
 
 	// Build labels for from/thread/reply-to/cc plus channel metadata
 	var labels []string