fix: Apply Sortable.Flex alignItems before items are measured

Sortable.Flex forced `alignSelf: 'flex-start'` on every item to prevent
stretching, but that also overrode a user-provided `alignItems` during
the initial (pre-measurement) render phase. With items of different sizes
this made them appear misaligned at first; they only snapped to the
correct alignment after measuring, causing a visible jump.

Only apply the `flex-start` override when `alignItems` is unset or
`stretch` (the cases where items would otherwise stretch). For any other
`alignItems` value, let it apply so items are aligned from the first
render.

Author: MatiPl01

packages/react-native-sortables/src/components/SortableFlex.tsx

@@ -235,15 +235,24 @@ function SortableFlexComponent({
     <SortableContainer
       {...rest}
       containerStyle={[baseContainerStyle, animatedContainerStyle]}
-      itemStyle={styles.styleOverride}
+      // `alignSelf: 'flex-start'` is only needed to prevent items from
+      // stretching (the default when no `alignItems` is set). When a
+      // specific `alignItems` is provided, let it apply so items are
+      // aligned from the first render instead of only after measuring.
+      itemStyle={
+        alignItems === undefined || alignItems === 'stretch'
+          ? styles.styleOverride
+          : undefined
+      }
       onLayout={runOnUI(handleContainerMeasurement)}
     />
   );
 }
 
 const styles = StyleSheet.create({
   styleOverride: {
-    // This is needed to prevent items from stretching (which is default behavior)
+    // Prevents items from stretching (the default flex behavior when no
+    // `alignItems` is set on the container)
     alignSelf: 'flex-start'
   }
 });