feat: Custom export dimensions

[ikorich]

Summary

Allow users to specify custom width/height for exports.

Motivation

Users may need specific dimensions for presentations or documentation.

Implementation

• Add width/height input fields
• Validate reasonable ranges
• Scale/fit content to dimensions

Files to Modify

• ToolBarView.swift - Dimension inputs
• ContentView.swift - Custom sizing logic

Acceptance Criteria

• Width/height fields in export section
• Content scales to fit dimensions
• Aspect ratio option (lock/unlock)
• Reasonable validation (min/max)

Story Points

2 points