Reader: compare button next to Aa

A button beside Aa in the reader's top bar opens a Compare translations
sheet (pick the Bible to compare with, show/hide the differing-word
highlights). It shows as active while a comparison is on. The controls
moved out of the Aa sheet; "C" opens it from the keyboard.

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

src/components/ShortcutsHelp.jsx

@@ -14,6 +14,7 @@ const BY_PAGE = {
     ['/', 'Search this chapter or the whole Bible'],
     ['G', 'Pick a book and chapter'],
     ['B', 'Bookmark the selected verse'],
+    ['C', 'Compare translations'],
     [',', 'Reading settings'],
     ['F', 'Hide or show the controls'],
   ],