v0.6.28

Invert the shortcut letter in each menu item

MenuItem now renders the label as Rich Text with the first letter matching its
shortcut key shown in reverse video (e.g. the R in "Record on/off", the Q in
"Quit"). Non-letter shortcuts like "?" and labels that don't contain the key
render unchanged. _mnemonic_index() is a small pure helper so the lookup is
unit-tested directly.

Menu titles have no keyboard open-shortcut, so only the item rows are
highlighted.

379 tests green; ruff + mypy clean.