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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.99.0"
+version = "0.100.0"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.99.0"
+version = "0.100.0"
 edition = "2021"
 default-run = "mangalord"