feat: Text callout annotations

[ikorich]

Summary

Add text callout boxes to explain code sections.

Motivation

Provide contextual explanations within the comparison image.

Implementation

• Add text boxes with configurable style
• Position boxes anywhere on panels
• Connect to code with lines
• Export includes callouts

Acceptance Criteria

• Add text callouts
• Position and resize callouts
• Callouts visible in export
• Edit/delete callouts

Story Points

5 points