style(icon): Improve icon sizing and rounded corners for macOS (#19)

The app icon appeared larger than other macOS apps in the Dock due to
insufficient padding and was being cut at the top/bottom by rounded corners.

Changes:
- Increased padding to 60% (content occupies ~62% of canvas)
- Applied rounded corners (~22% radius) matching macOS design guidelines
- Fixed rendering pipeline to avoid cutting the "A" logo
- Icon now has proportions consistent with system apps (Chrome, Spotify, etc.)

The icon rendering process now:
1. Rounds the original icon first
2. Adds transparent padding
3. Composites on colored background
4. Applies final rounded corners to full canvas

This prevents the logo from being cut while maintaining proper visual size.

Author: wilcorrea