fix: implement macOS shortcut creation in ShortcutHelper

[mock1ngbb]

ShortcutHelper.CreateGameShortcut() and related methods throw PlatformNotSupportedException on macOS:

MainWindow.axaml.cs line 5860 calls CreateGameShortcut
Services/ShortcutHelper.cs line 37 throws for macOS shortcuts
Services/ShortcutHelper.cs line 52 throws for non-Steam shortcuts on non-Windows/Linux

Scope:

1. Implement macOS shortcut creation via .webloc or Automator .app wrapper
2. Implement Steam shortcut addition for macOS (parse shortcuts.vdf)
3. Test on actual macOS/arm64 hardware

AXIOM 3 (Loud-by-default): PlatformNotSupportedException with no tracked path forward is a hard fail-loud.
AXIOM 1 (Hardened): a cross-platform launcher should work on its development platform (macOS arm64).