fix: regenerate app icon as solid RGB PNG via AppKit

Previous icon had transparent corners (RGBA with rounded mask) which
rendered as black on iOS. Replaced with solid RGB square using AppKit
NSAttributedString to render 🔐 emoji at 720pt on dark gradient.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: heyfinal