fix: properly dispose panel and group event subscriptions in effect cleanup

[iXanadu13]

What

Wrap disposables.forEach(disposable => disposable.dispose()) in the useEffect cleanup function instead of executing it immediately.

Why

The disposal logic was executed directly in the effect body, which caused all previously registered listeners to be disposed right after registration, making them ineffective.

Demo

https://codesandbox.io/p/sandbox/nostalgic-tdd-ypjjdv

[codesandbox-ci]

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externaldnd-dockview	Configuration
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keyboard-dockview	Configuration
nativeapp-dockview	Configuration
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