feat: Add file chooser support for widget preview initialization.

[nopjar]

When using a multi-module project, the widget previewer sometimes fails to automatically find the correct pubspec.yaml. Adding the possibility to manually choose the file allows for more complex project setups.

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[gemini-code-assist]

Code Review

This pull request enables users to specify the pub root module for the Flutter Widget Previewer by adding a file selection dialog and a retry mechanism when the root is not automatically detected. Review feedback highlights the need to wrap file system and project index access within Read Actions to comply with IntelliJ threading requirements and suggests externalizing user-facing strings for localization.

[pq]

Fantastic! Since Helin wrote this, I'll defer to her for a review when she's back from vacation.

[helin24]

LGTM! Thanks for the contribution.

Can you add a screenshot of the file choosing dialog?

[nopjar]

Sure! I've added your suggested change and added a screenshot of the file chooser dialog to the PR description.

[helin24]

Thanks @nopjar!