Add macOS packaging with .dmg and .app bundle

- Add cargo-bundle configuration for macOS app bundle
- Create placeholder app icon (blue with "A" letter)
- Update build-all.sh to create .dmg and .app.zip
- Update README with new build output table

macOS packages:
- .dmg: Disk image for drag-to-Applications install
- .app.zip: Zipped app bundle alternative

Cargo.toml

@@ -55,3 +55,18 @@ assets = [
 ]
 [package.metadata.generate-rpm.requires]
 ffmpeg = "*"
+
+[package.metadata.bundle]
+name = "AVIF Maker"
+identifier = "com.cottongin.avif-maker"
+icon = ["resources/AppIcon.icns"]
+version = "0.1.0"
+copyright = "Copyright © 2026 cottongin. All rights reserved."
+category = "public.app-category.graphics-design"
+short_description = "Convert images and videos to AVIF format"
+long_description = """
+AVIF Maker is a native desktop application for converting images and videos 
+to the AVIF format. It supports PNG, JPEG, WebP, GIF (animated), and video 
+files. Features include quality settings, speed presets, and batch processing.
+"""
+osx_minimum_system_version = "10.14"