fix(ui): mobile viewport, scrollable popovers, and actor labels

- Set viewport-fit=cover and disable user scaling for mobile PWA feel
- Wrap assignee/project popover lists in scrollable containers
- Remove rounded-t-sm from stacked chart bars for cleaner rendering
- Prevent filter bar icons from shrinking on narrow screens
- Show "Board" instead of raw user IDs in activity feeds and toasts
- Surface server error message in health API failures

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

ui/src/pages/IssueDetail.tsx

@@ -110,7 +110,8 @@ function ActorIdentity({ evt, agentMap }: { evt: ActivityEvent; agentMap: Map<st
     return <Identity name={agent?.name ?? id.slice(0, 8)} size="sm" />;
   }
   if (evt.actorType === "system") return <Identity name="System" size="sm" />;
-  return <Identity name={id || "You"} size="sm" />;
+  if (evt.actorType === "user") return <Identity name="Board" size="sm" />;
+  return <Identity name={id || "Unknown"} size="sm" />;
 }
 
 export function IssueDetail() {