Build and mount the keyboard-shortcut help overlay, rendering the grouped
shortcut sections and closing tips defined in this module.
Idempotent: if an overlay is already open it returns immediately rather than
stacking a second one. Sets up a focus trap and a delayed
click-outside-to-dismiss handler (the delay avoids catching the same click
that opened it); hideHelpOverlay tears both down.
Build and mount the keyboard-shortcut help overlay, rendering the grouped shortcut sections and closing tips defined in this module.
Idempotent: if an overlay is already open it returns immediately rather than stacking a second one. Sets up a focus trap and a delayed click-outside-to-dismiss handler (the delay avoids catching the same click that opened it); hideHelpOverlay tears both down.