Fix scrolling issues in CupertinoPicker

[ndonkoHenri]

Fixes #3649

Notes

• I don't have Windows, so can't tell if it will eventually fix the issue on that platform. Needs testing on Windows.
• CupertinoTimerPicker was reported to have thesame issue, but it has no scrollController, hence the SO solution can't be applicable.

Test Code

https://flet.dev/docs/controls/cupertinopicker

Summary by Sourcery

This pull request addresses and fixes the scrolling issues in the CupertinoPicker component by introducing a FixedExtentScrollController and adding logic to manage scroll events, ensuring correct item selection and smooth scrolling behavior.

• Bug Fixes:
  • Resolved scrolling issues in CupertinoPicker by implementing a FixedExtentScrollController and managing scroll events to ensure proper item selection.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request addresses the scrolling issues in CupertinoPicker by introducing a FixedExtentScrollController and adding logic to manage scroll events. The changes include initializing the scroll controller with the selected index, adding a listener to handle scroll direction, and updating the build method to use the scroll controller.

File-Level Changes

Files	Changes
packages/flet/lib/src/controls/cupertino_picker.dart	Introduced a FixedExtentScrollController to manage the scroll position and added logic to handle scroll events to fix the scrolling issues in CupertinoPicker.

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[sourcery-ai]

Hey @ndonkoHenri - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟡 General issues: 1 issue found
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[sourcery-ai]

suggestion: Consider using setState when updating _index.

To ensure the UI updates correctly, consider wrapping the assignment of _index in a setState call.

Suggested change

	_index = index;
	setState(() {
	_index = index;
	});