feat: 3+ panel support

[ikorich]

Summary

Support more than 2 code panels for Bad/Good/Better comparisons.

Motivation

Users want to show progression or multiple alternatives.

Implementation

• Dynamic panel count (2-4)
• Custom indicator for each panel
• Layout adapts to panel count
• Export handles multiple panels

UI Considerations

• Add panel button
• Remove panel button
• Horizontal scroll for many panels

Acceptance Criteria

• Add up to 4 panels
• Each panel has custom indicator
• Layout adjusts automatically
• Export includes all panels

Story Points

8 points