With .ykanban present but ykanban.db gone, opening used to create an empty
database and show an empty board without any hint. It now throws
WorkspaceDatabaseMissingException and shows the open-failed page, telling the
user to restore a backup or delete .ykanban and initialize again.
- 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