Snackbar stays mounted after `visible=false` under new architecture (iOS + Android)

## Summary

Under React Native's new architecture (Fabric), paper's `` can get stuck mounted at non-zero opacity after `visible` is set to `false`. The hide animation runs visually, but the component never unmounts and stays visible on screen until something else forces a remount.

Sister bug to #3775 / #4445 (which were Android-only show path, fixed by #4447). This is the **hide path**, and reproduces on **both iOS and Android** with the new arch enabled.

## Root cause

[`src/components/Snackbar.tsx`](https://github.com/callstack/react-native-paper/blob/main/src/components/Snackbar.tsx#L219-L223), `handleOnHidden`:

```tsx
Animated.timing(opacity, {
toValue: 0,
duration: 100 * scale,
useNativeDriver: true,
}).start(({ finished }) => {
if (finished) { // ← this guard is the problem under Fabric
setHidden(true);
}
});
```

Under Fabric, this `start({finished})` callback fires with `finished: false` even when nothing has interrupted the animation. `setHidden(true)` is the only thing that makes the component return `null` and actually unmount, so the snackbar stays rendered at whatever opacity the (visually-running) animation reached.

The parallel show-path fix #4447 split `handleOnVisible` from `animateShow` to work around the same Fabric noise. The hide path was not touched and still carries the guard.

## Expected behaviour

After `visible={false}`, the Snackbar component unmounts cleanly (`hidden` flips to `true`) once the hide animation completes, regardless of the `finished` flag.

## Current behaviour

After `visible={false}`:
- The hide animation fires (opacity 1 → 0).
- The `start` callback fires with `finished: false`.
- `setHidden(true)` is skipped.
- The component stays mounted; nothing in the React state can clear it.
- Subsequent `setVisible(false)` taps re-trigger the same broken animation; the snackbar never goes away.

## Repro

1. New arch enabled.
2. Mount a global ` setVisible(false)} action={{ label: 'Action', onPress: () => {} }}>Message`.
3. `setVisible(true)`, then `setVisible(false)`.
4. Snackbar stays on screen.

Reproduced on:
- iOS 18 simulator (iPhone 15)
- iOS 26 simulator (iPhone 17 Pro Max)
- Android Pixel 8 emulator (API 35)

All with `newArchEnabled: true`.

## Environment

- `react-native-paper`: 5.15.2
- `react-native`: 0.83.6
- Expo SDK 55
- Fabric / new arch: enabled

## Proposed fix

Drop the `if (finished)` guard, mirroring how #4447 worked around the same class of Fabric noise on the show path:

```diff
Animated.timing(opacity, {
toValue: 0,
duration: 100 * scale,
useNativeDriver: true,
- }).start(({ finished }) => {
- if (finished) {
- setHidden(true);
- }
+ }).start(() => {
+ setHidden(true);
});
```

### Trade-off

Under a *genuine* interrupt (a new show kicked off mid-hide), `setHidden(true)` would now also fire. The next render uses the new `visible=true` path → `setHidden(false)` → re-animates show. The visible artefact is at worst a one-frame snap. A stuck Snackbar that no state change can clear is strictly worse.

Happy to open a PR if useful. Tested locally via `patch-package`; resolves the issue on both platforms.

Read src/components/Snackbar.tsx, focusing on handleOnHidden and the hide animation callback; compare its behavior with the show-path workaround in #4447. Reproduce with Fabric enabled by toggling visible from true to false on iOS or Android. Done means the hide animation leaves the Snackbar unmounted and hidden even when the callback reports finished: false.

索引モデルが issue の本文から書いたものです。