Improve handling of existing citation files during import

## 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

The issue names the GitHub importer and citation metadata UI as entry points; inspect how existing and generated values are currently detected and presented. Document the desired import behavior and identify UI changes for conflicts, source visibility, warnings, and choosing values. Done means the import behavior is documented and the UI improvements are identified.

Written by the indexing model from the issue text.