refactor: simplify UNSORTED_SAVED_AT sort to use createdAt directly

[lcomplete]

Summary

• Replace complex multi-field fallback sorting (collectedAt -> savedAt -> archivedAt) with simpler createdAt-based sorting for unsorted pages
• Ensure collectedAt is always set when pages are added to library, preventing null values in batch operations
• Simplify PageListSort enum by removing multi-field sort logic

Test plan

• Verify unsorted pages list displays correctly sorted by creation time
• Test batch move operations to ensure collectedAt is properly set
• Confirm regular collection sorting (COLLECTED_AT) still works as expected

🤖 Generated with Claude Code

[augmentcode]

🤖 Augment PR Summary

Summary: Refactors “unsorted” page sorting to use a single, always-present timestamp (createdAt) instead of a multi-field fallback.

Changes:

• Updates client sort labeling so UNSORTED_SAVED_AT is shown as “Created”
• Simplifies PageListSort by removing multi-field sort support and using createdAt for unsorted pages
• Updates page item mapping/pagination record timestamp for unsorted lists to use createdAt
• Ensures collectedAt is populated via batch operations and like-to-library flows by coalescing to createdAt (or publish time where applicable)

Technical Notes: Server-side sort building in PageListService is simplified to a single-field sort now that unsorted no longer requires multi-field ordering.

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[augmentcode]

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