Fix chat comment alignment, avatars, and layout polish

- Agent messages: avatar outside left (matching feed items alignment),
  always shown, consistently uses icon avatar instead of initials
- User messages: avatar outside right, action bar moved below the
  gray bubble, gray darkened to bg-muted
- System events: right-aligned when actor is the current user
- Run messages: use agent icon avatar consistently
- Pass actorType/actorId in event metadata for current-user detection

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/lib/issue-chat-messages.ts

@@ -235,6 +235,8 @@ function createTimelineEventMessage(args: {
         anchorId: `activity-${event.id}`,
         eventId: event.id,
         actorName,
+        actorType: event.actorType,
+        actorId: event.actorId,
         statusChange: event.statusChange ?? null,
         assigneeChange: event.assigneeChange ?? null,
       },