fix: Respect Sortable.Flex alignItems before items are measured (#560)

## Summary

`Sortable.Flex` put `alignSelf: 'flex-start'` on every item to stop them
stretching — stretched items get measured at the wrong size, which
breaks the absolute layout (originally added in the "flex layout
flickering" fix, #258). But `alignSelf` on an item **overrides the
container's `alignItems`**, so any `alignItems` the user set (e.g.
`center`) was silently ignored during the initial (pre-measurement)
render and only took effect once the component switched to its absolute
layout — a visible "snap" jump, most noticeable with differently-sized
items.

Stretch-prevention belongs on the **container**, not on each item. This
drops the per-item `alignSelf` and instead falls back to `alignItems:
'flex-start'` on the container **only** when no specific `alignItems` is
set (or it's `stretch`). Any explicit `alignItems` now applies from the
first render, with no jump.

## Repro

`<Sortable.Flex alignItems="center">` with children of different heights
— before: items start top-aligned and snap to centered after the
measurement gate. After: centered immediately.

## Notes

The default (no `alignItems`) behaviour is unchanged — items still don't
stretch and stay top-aligned. The absolute-layout (post-measurement)
path already handled `alignItems` correctly and is untouched. Worth a
quick visual check in the example app's Sortable Flex screens.

Author: MatiPl01

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

@@ -1,6 +1,5 @@
 import { memo, useMemo } from 'react';
 import type { ViewStyle } from 'react-native';
-import { StyleSheet } from 'react-native';
 import { runOnUI, useAnimatedStyle } from 'react-native-reanimated';
 
 import { DEFAULT_SORTABLE_FLEX_PROPS } from '../constants';
@@ -224,7 +223,8 @@ function SortableFlexComponent({
         }
       : {
           alignContent,
-          alignItems,
+          // Avoid the default `stretch` so items keep their measured size
+          alignItems: alignItems ?? 'flex-start',
           flexDirection,
           flexWrap,
           justifyContent
@@ -235,17 +235,9 @@ function SortableFlexComponent({
     <SortableContainer
       {...rest}
       containerStyle={[baseContainerStyle, animatedContainerStyle]}
-      itemStyle={styles.styleOverride}
       onLayout={runOnUI(handleContainerMeasurement)}
     />
   );
 }
 
-const styles = StyleSheet.create({
-  styleOverride: {
-    // This is needed to prevent items from stretching (which is default behavior)
-    alignSelf: 'flex-start'
-  }
-});
-
 export default SortableFlex;