Add long citation key detection with user guidance notification.

[u7978428]

Summary

Adds long citation key detection with user guidance notifications.

Changes

• Detect citation keys longer than 20 characters during import
• Show user-friendly notification with guidance message
• Add logging for detection events
• Integrate into ImportHandler workflow

Impact

Helps users understand when shorter citation keys might improve readability and management.

[SLin417]

No need to do that, already handled using CitationKeyGenerator to generate new keys of desired format.

[Junqi597]

Solid improvement — very thoughtful touch adding user guidance.
Clear logic and clean integration, fits naturally in the import flow.
Nice to see proper logging and threshold control.