Restore notebook detachment pipeline

[MelkorBalrog]

Summary

• Re-enable ClosableNotebook tab detachment using the existing transfer pipeline so tabs move into floating windows with lifecycle hooks intact
• Add defensive configuration copying helper and cleanup hooks for detached tabs while keeping failure logging guarded
• Extend GUI regression coverage and pytest markers, documenting the detachment restoration in HISTORY

Testing

• pytest (fails: external dependency Pillow is unavailable in the execution environment, causing governance and splash screen imports to error)

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