Rewrite library: native SF Symbol rendering, drop legacy support

- Render app.fill at natural size with draw(at:) instead of draw(in:) to preserve squircle shape
- Add SettingsIconSize enum with standard (32/16pt) and custom sizes
- Use UIGraphicsImageRenderer instead of deprecated UIGraphicsBeginImageContext
- SwiftUI view uses .resizable().scaledToFit() for correct proportions
- Bump to Swift 6.0, iOS 15+, macOS 13+
- Remove CocoaPods support, GitHub templates, and community docs
- Add .build to .gitignore

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Update README.md

Clean code and fixed font styles.

Update SPSettingsIcons.podspec