Fix user provided scroll controllers not being managed correctly

[pitriq]

Description

Hey,

I noticed that when passing a custom ScrollController to one of my sheet’s pages, it was being attached to multiple scroll views, causing an error. The issue was due to a small mistake in the code that recreates scroll controllers when pages are initialized or changed.

This PR ensures that when scroll controllers are recreated inside WoltModalSheetAnimatedSwitcher, user-provided controllers (if present) are correctly reused. It also avoids disposing them, as they're user managed.

Additionaly, I fixed the offset-tracking logic to properly track the offset of each controller — previously, it was repeatedly tracking the same controller's offset.

Related Issues

I believe one of the problems mentioned in #350 may be related to this.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes tests for all changed/updated/fixed behaviors.
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.