`tuiapp_v2.py:27` imports `fmt_key`, `fmt_keys` from `keysym`, but the
module file was never tracked. PR #513 (tui v2/v3 fixes) is unusable on a
fresh checkout without this. Adds the cross-platform shortcut formatter
that turns binding strings into mac/Win-Linux key labels (e.g.
`"ctrl+b"` → `"⌃B"` / `"Ctrl+B"`), honoring `GA_KEYSYM_STYLE`.