feat: add macOS-specific app icon with full-canvas design

Same terminal session content as Linux/Windows (echo, cterm --version,
version output, prompt with block cursor) but without window chrome.
macOS applies its own rounded rect mask so the icon fills the full
space instead of appearing as a small image inside the system shape.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MagicalTux

.github/workflows/build.yml

@@ -619,7 +619,7 @@ jobs:
           VERSION=$(grep '^version = ' Cargo.toml | head -1 | sed 's/version = "\(.*\)"/\1/')
           echo "Building icon for version: $VERSION"
 
-          # Use macOS-specific icon template (no window chrome, fills full canvas)
+          # Use macOS-specific icon template
           sed "s/{{VERSION}}/$VERSION/g" packaging/macos/icon_template.svg > /tmp/cterm_icon.svg
 
           # Create iconset directory