feat(web): 'a' keyboard shortcut toggles the annotations drawer

The drawer was the only top-bar tool without a keyboard shortcut, so the
shortcuts & gestures help modal had to describe it as '📝 button'. Bind
plain 'a' to toggleBookmarks(), following the same pattern as the
existing t/s/b letter shortcuts.

- reader.ts: new 'a' branch in handleKeydown, guarded against
  Ctrl/Cmd/Alt so Select All (a common reader action when selecting
  text to highlight/copy) is never intercepted; typing guard already
  protects note textareas, search box, and selects. Works in both the
  host page and book-content iframes since both keydown listeners route
  through handleKeydown.
- reader.templ: help modal row now shows <kbd>a</kbd> like the other
  letter shortcuts; 📝 button tooltip gains '(a)' to match its
  siblings (b), (/), (s), (?).
- reader_templ.go: regenerated.

web/src/reader/reader.ts

@@ -3535,6 +3535,9 @@ document.addEventListener("alpine:init", () => {
         if (k === "t") this.toggleTOC();
         else if (k === "s") this.toggleSettings();
         else if (k === "b") this.addBookmark();
+        else if (k === "a") {
+          if (!event.ctrlKey && !event.metaKey && !event.altKey) this.toggleBookmarks();
+        }
         else if (k === "?") this.toggleHelp();
         else if (k === "/") {
           event.preventDefault();