[BE-14] Add support for loading saved editing sessions

[nmy2103]

Summary

We'd like to add support for exporting and loading editing sessions in the Beat Editor. This feature will allow users to restore their data cleaning and annotation progress from a file. Users should be able to shut down the application and later resume work in a new instance without losing progress.

Acceptance Criteria

• Users can export an editing session
• Users can select and load a previously saved session file
• The application restores all edits on the uploaded data
• The restored session matches the state at the time of saving
• Loading a session handles invalid files

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Reviewers: @nmy2103; @ynwtnb