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.

templates/reader.templ

@@ -277,7 +277,7 @@ templ Reader(user User, metadata ReaderMetadata) {
 					<div class="help-row"><span>Page edge zones <span class="help-note">(desktop)</span></span><span class="help-keys">click page edges</span></div>
 					<div class="help-row" x-show="!isFixedLayout || isPDF"><span>Search in book</span><span class="help-keys"><kbd class="kbd">/</kbd></span></div>
 					<div class="help-row"><span>Contents / thumbnails</span><span class="help-keys"><kbd class="kbd">t</kbd></span></div>
-					<div class="help-row"><span>Annotations drawer</span><span class="help-keys">📝 button</span></div>
+					<div class="help-row"><span>Annotations drawer</span><span class="help-keys"><kbd class="kbd">a</kbd></span></div>
 					<div class="help-row"><span>Settings</span><span class="help-keys"><kbd class="kbd">s</kbd></span></div>
 					<div class="help-row"><span>Add bookmark</span><span class="help-keys"><kbd class="kbd">b</kbd></span></div>
 					<div class="help-row"><span>Close menus / toggle chrome</span><span class="help-keys"><kbd class="kbd">Esc</kbd></span></div>
@@ -311,7 +311,7 @@ templ ReaderChrome(metadata ReaderMetadata) {
 				<div class="flex items-center gap-1">
 					<button @click="addBookmark()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" title="Bookmark this position (b)">🏷️</button>
 					<button x-show="(!isFixedLayout || isPDF) && !treatAsComic" @click="toggleSearch()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" title="Search (/)">🔍</button>
-					<button @click="toggleBookmarks()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" title="Annotations">📝</button>
+					<button @click="toggleBookmarks()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" title="Annotations (a)">📝</button>
 					<button @click="toggleSettings()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700" title="Settings (s)">Aa</button>
 					<button @click="toggleHelp()" class="p-1.5 sm:p-2 rounded-lg hover:bg-gray-700 text-sm font-bold" title="Shortcuts & gestures (?)" aria-label="Shortcuts and gestures">?</button>
 				</div>