The window, the Windows executable and the macOS bundle now use a
placeholder kanban icon (three columns of cards) instead of the template's
avalonia-logo.ico, which had no icon at all on macOS.
- macOS: document open -n -a with an absolute path (plan updated accordingly)
- package.sh: zip with 7z for '/' entry separators; ad-hoc codesign failure is a warning
- USAGE: list blank and unrepresentable paths as invalid arguments