feat: Line highlighting

[ikorich]

Summary

Highlight specific lines with background color to draw attention.

Motivation

Users want to emphasize particular lines of code in comparisons.

Implementation

• Add line selection mode
• Apply highlight background to selected lines
• Configurable highlight color
• Persist highlight state per panel

Files to Modify

• CodePanel.swift - Track highlighted lines
• CodeEditorView.swift - Render highlights
• ToolBarView.swift - Highlight controls

Acceptance Criteria

• Can select lines to highlight
• Highlight visible in editor
• Highlight appears in export
• Clear highlights option

Story Points

3 points