Polish issue chat actions and overflow

- Scale activity components (events, runs) to ~80% font size with
  xs avatars for a quieter visual weight
- Hide succeeded runs from the timeline; only show failed/errored
- Always show three-dots menu on agent comments with "Copy message"
  option, plus optional "View run" when available
- User avatar repositioned to top-right (items-start) of message
- Change "Me" → "You" in assignee labels for natural chat phrasing
  ("You updated this task")

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

ui/src/lib/assignees.ts

@@ -76,7 +76,7 @@ export function formatAssigneeUserLabel(
   currentUserId: string | null | undefined,
 ): string | null {
   if (!userId) return null;
-  if (currentUserId && userId === currentUserId) return "Me";
+  if (currentUserId && userId === currentUserId) return "You";
   if (userId === "local-board") return "Board";
   return userId.slice(0, 5);
 }