Reader: show how much another translation differs from the one being read

- Word-level comparison (ignores case and punctuation): differing words are
  marked in both columns of the side-by-side view
- Chapter summary ("NET is 18% different from BSB · 37 of 39 verses differ")
  with links to the most different verses, and a percentage on each verse
- Compare tab on a verse shows each translation's difference from the one
  being read, with the differing words marked
- "Highlight differences" setting; your own highlights and search matches
  always win where they overlap a difference mark

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

src/hooks/useBibleReader.js

@@ -103,6 +103,7 @@ export function useBibleReader(currentPage, translationOverride = null) {
   const [readerFont, setReaderFont] = useStoredChoice('reader-font', 'serif', READER_FONTS);
   const [readerSpacing, setReaderSpacing] = useStoredChoice('reader-spacing', 'normal', READER_SPACINGS);
   const [readerVerseNumbers, setReaderVerseNumbersRaw] = useStoredChoice('reader-verse-numbers', 'on', ['on', 'off']);
+  const [readerShowDiff, setReaderShowDiffRaw] = useStoredChoice('reader-show-diff', 'on', ['on', 'off']);
   const [readerParallel, setReaderParallel] = useStoredChoice('reader-parallel', '', ['', ...READER_TRANSLATIONS.map((t) => t.id)]);
   // The page the app opens to, and where every Home button goes: 'studies' | 'reader' | 'plans'
   const [startPagePref, setStartPagePrefState] = useState(() => {
@@ -532,6 +533,7 @@ export function useBibleReader(currentPage, translationOverride = null) {
     readerSpacing, setReaderSpacing,
     readerVerseNumbers: readerVerseNumbers === 'on', setReaderVerseNumbers: (on) => setReaderVerseNumbersRaw(on ? 'on' : 'off'),
     readerParallel, setReaderParallel,
+    readerShowDiff: readerShowDiff === 'on', setReaderShowDiff: (on) => setReaderShowDiffRaw(on ? 'on' : 'off'),
     planTranslationPref, setPlanTranslationPref,
     startPagePref, setStartPagePref,
     readerBookmarks, setReaderBookmarks,