PPR-26: Book import workflow rework

[Eoic]

Ticket

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Summary

Describe the problem, what changed, and why this approach was chosen.

Testing

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UI evidence

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Compatibility and migration

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Notes

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Checklist

• The branch, commits, and pull request title follow the repository conventions.
• The change is complete and limited to the ticket's scope.
• Relevant tests have been added or updated.
• Formatting, analysis, and tests pass.
• Documentation, UI evidence, and migration notes are included where relevant.
• No credentials, personal data, copyrighted fixtures, or unrelated generated files are included.
• The branch is up to date with its target branch.

[Copilot]

Pull request overview

Adds a design/spec document describing a reworked book import workflow, separating digital/physical import into distinct bottom sheets and introducing a multi-file digital import flow with a results/commit step.

Changes:

• Documented new bottom-sheet structure (AddBookSheetScaffold) to standardize fixed header/body/footer layout across add-book flows.
• Specified a multi-file digital import selection sheet (DigitalBookImportSheet) and a processing/commit results sheet (BookImportResultsSheet) with retry/removal and cleanup rules.
• Outlined verification coverage expectations (widget/model tests) and operational assumptions for the workflow.

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