fix(mobile): infer drag intent early (#26344)

The drag intent was not set until it reached the kTouchSlop threshold.
This is not necessary as flutter already has its own heuristics for
preventing unintended drags.

The result of using kTouchSlop is that dismissing or scroll can feel a
little delayed, and will jump from 0 to kTouchSlop (18px) rather than
moving smoothly.

mobile/lib/presentation/widgets/asset_viewer/asset_page.widget.dart

@@ -143,8 +143,8 @@ class _AssetPageState extends ConsumerState<AssetPage> {
 
     if (_dragIntent == _DragIntent.none) {
       _dragIntent = switch ((details.globalPosition - _dragStart!.globalPosition).dy) {
-        < -kTouchSlop => _DragIntent.scroll,
-        > kTouchSlop => _DragIntent.dismiss,
+        < 0 => _DragIntent.scroll,
+        > 0 => _DragIntent.dismiss,
         _ => _DragIntent.none,
       };
     }