Add keyboard shortcuts help overlay (press ?)

[Munan059]

Pressing ? toggles a dismissable overlay listing the keyboard map from the README, styled to the current theme.

What changed

• editor.js: _keyDown emits a help event on ? (Shift+/).
• ui.js: adds the overlay, built inside the UI layer (.qd-ui) and toggled via editor.on('help', ...). Closes on Esc / ? / backdrop click / close button, and is torn down in destroy().
• quickdraw.css: theme-styled .qd-help-* rules reusing the existing --qd-* tokens.

The overlay lives in the UI layer so hideUi hosts are unaffected.

Closes #3

[vercel]

@Munan059 is attempting to deploy a commit to the naman's projects Team on Vercel.

A member of the Team first needs to authorize it.