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.test.ts

@@ -47,7 +47,7 @@ describe("assignee selection helpers", () => {
   });
 
   it("formats current and board user labels consistently", () => {
-    expect(formatAssigneeUserLabel("user-1", "user-1")).toBe("Me");
+    expect(formatAssigneeUserLabel("user-1", "user-1")).toBe("You");
     expect(formatAssigneeUserLabel("local-board", "someone-else")).toBe("Board");
     expect(formatAssigneeUserLabel("user-abcdef", "someone-else")).toBe("user-");
   });