Improve handling of existing citation files during import

[beanbean9339]

Summary

Improve the GitHub importer when existing citation metadata is detected.

Motivation

Repositories may already contain manually curated citation metadata that should not be lost.

Possible Improvements

• Display import source for each field
• Warn when imported values differ
• Allow users to choose between existing and generated values
• Highlight conflicting metadata
• Preserve manual edits whenever possible

Acceptance Criteria

• Desired import behavior is documented
• UI improvements are identified