Update App icon to macOS modern style

[schuyler]

Pull Request

Based on PR #1355 from MacDownApp/macdown by @fisher158163
MacDownApp/macdown#1355

Description

Updated the App icon to the macOS 26 Liquid Glass style.

Benefits

• Modern, updated appearance consistent with macOS design language
• Better visual integration with macOS Big Sur and later
• Professional appearance for MacDown 3000

Implementation Notes

• Review the icon design from the PR
• Ensure icon works at all required sizes (16x16 to 512x512)
• Test appearance in Dock, Finder, and application switcher
• Verify icon meets macOS Human Interface Guidelines
• Consider if we want to adopt this design or create a unique MacDown 3000 icon

Priority

LOW - Cosmetic enhancement

Labels

enhancement, design

[schuyler]

relates to #340