Feature: Directly overwrite existing highlight color

[cuspymd]

Summary

Changing an existing highlight color is cumbersome because it requires delete-and-reapply.

Problem

• Users must remove an existing highlight before changing its color.
• Editing becomes slower and repetitive.

Request

• Allow immediate color replacement when reselecting the same or partially overlapping range.
• Provide a UX that supports color changes without a separate delete step.

Expected Outcome

• Faster highlight editing
• Fewer steps and better usability for color changes