Reader: Highlights button in the top bar

A highlighter button beside Bookmarks opens your highlights (by colour);
Bookmarks now opens the bookmarks. Each shows in the accent colour when you
have some. "H" opens the highlights.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/components/ShortcutsHelp.jsx

@@ -15,6 +15,7 @@ const BY_PAGE = {
     ['G', 'Pick a book and chapter'],
     ['B', 'Bookmark the selected verse'],
     ['C', 'Compare translations'],
+    ['H', 'Your highlights'],
     [',', 'Reading settings'],
     ['F', 'Hide or show the controls'],
   ],