feat(reader): add a keyboard-shortcut help overlay

Press `?` in the reader to open a Sheet documenting the existing bindings
(arrows/J/K paging, Home/End, chapter nav, Esc); Esc or the close button
dismisses it. While open, the reader swallows navigation keys so paging
can't happen behind the overlay. e2e covers open-on-? and close-on-Esc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.99.0",
+  "version": "0.100.0",
   "private": true,
   "type": "module",
   "scripts": {