feat: add TouchSensor for mobile drag and drop compatibility

Implement dnd-kit TouchSensor with proper activation constraints (250ms delay, 8px tolerance) to enable drag and drop functionality on mobile devices.

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

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

apps/web/src/components/EnhancedChampionArena.tsx

@@ -7,6 +7,7 @@ import {
   closestCenter,
   KeyboardSensor,
   PointerSensor,
+  TouchSensor,
   useSensor,
   useSensors,
   DragStartEvent,
@@ -385,6 +386,12 @@ export function EnhancedChampionArena({ onGameModeChange, onConfigurePlayer, cla
         distance: 8,
       },
     }),
+    useSensor(TouchSensor, {
+      activationConstraint: {
+        delay: 250,
+        tolerance: 8,
+      },
+    }),
     useSensor(KeyboardSensor, {
       coordinateGetter: sortableKeyboardCoordinates,
     })