fix(card-sorting): hide activity notifications in spectator mode

The activity feed showing "player X is moving cards" is unnecessary
in observation mode since spectators can see the actions happening
on the game board.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/arcade-games/card-sorting/components/PlayingPhaseDrag.tsx

@@ -1909,8 +1909,8 @@ export function PlayingPhaseDrag() {
         })}
       </div>
 
-      {/* Activity Feed (collaborative mode only) */}
-      {state.gameMode === 'collaborative' && activityFeed.length > 0 && (
+      {/* Activity Feed (collaborative mode only, hidden for spectators) */}
+      {state.gameMode === 'collaborative' && !isSpectating && activityFeed.length > 0 && (
         <div
           className={css({
             position: 'fixed',