feat: Arrow annotations

[ikorich]

Summary

Add arrow annotations to point to specific code sections.

Motivation

Visual emphasis helps explain what's wrong/right.

Implementation

• Drawing mode for arrows
• Arrow customization (color, style)
• Arrows persist with comparison
• Export includes annotations

Acceptance Criteria

• Draw arrows on code panels
• Customize arrow appearance
• Arrows visible in export
• Delete/edit arrows

Story Points

5 points