[D3-04] Import Zotero library without duplicate or data loss

[dkritarth]

Outcome

Import selected Zotero items and attachments into Vellum with predictable deduplication and provenance.

Dependencies

Blocked by #45.

Scope

Research supported local/export integration first. Preview import, metadata mapping, attachment copy, deduplication, collections mapping, error recovery, and import report. No credential extraction or cloud account automation without separate approved design.

Acceptance criteria

• User previews selected items and destination before import.
• DOI/arXiv/attachment identity deduplicates predictably.
• Metadata and attachment provenance are recorded.
• Existing Vellum notes/chats/highlights are never overwritten.
• Missing attachments and malformed exports produce actionable report.
• Re-running import is idempotent.
• Cancel/partial failure leaves consistent state.

Live verification

Import representative export with duplicates, nested collections, missing PDF, Unicode metadata, and existing Vellum paper; rerun, cancel, restart, inspect report/files/DB.

Required evidence

Focused PR uses Closes #46. Provide a written design first, failing tests/baseline, focused/full tests, typecheck, build, diff check, exact live Electron procedure, real-paper evidence, screenshots, console state, performance observations, and limitations. Independent reviewer repeats representative workflow.