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/components/reader/ReaderText.jsx

@@ -1,6 +1,9 @@
 // The page itself: chapter heading, translation notice, and verses set in a
 // serif face. Tap a verse to open the verse sheet; select text to highlight.
+import { useMemo } from 'react';
 import { formatCrossRef } from '../../utils/wordStudy.js';
+import { diffChapter, differenceLabel } from '../../utils/textDiff.js';
+import { DiffText } from './DiffText.jsx';
 
 // How each typeface choice is set. "Easy to read" is a sans face with extra letter and word spacing.
 const FONT_STYLES = {
@@ -13,10 +16,18 @@ const LINE_HEIGHTS = { snug: 1.6, normal: 1.85, airy: 2.15 };
 // `parallel` ({ translation, name, notice, verses, loading, error }) shows a second Bible beside the first.
 export default function ReaderText({
   title, translation, translationId, notice, verses, loading, error, layout, fontSize, wide,
-  font = 'serif', spacing = 'normal', showVerseNumbers = true, parallel = null,
+  font = 'serif', spacing = 'normal', showVerseNumbers = true, parallel = null, showDiff = true,
   bookmarks, bookKey, chapter, selectedVerse, onSelectVerse, renderVerseText, searchLower,
   crossRefs, showCrossRefs, onSelectionEnd, footer,
 }) {
+  // How much the second Bible's wording differs from the first, over the chapter and verse by verse
+  const diff = useMemo(
+    () => (parallel && showDiff && !parallel.loading && !parallel.error && parallel.verses?.length ? diffChapter(verses, parallel.verses) : null),
+    [parallel, showDiff, verses],
+  );
+  const diffByVerse = useMemo(() => new Map((diff?.verses ?? []).map((d) => [d.number, d])), [diff]);
+  const jumpToVerse = (n) => document.getElementById(`reader-verse-${n}`)?.scrollIntoView?.({ block: 'center', behavior: window.matchMedia?.('(prefers-reduced-motion: reduce)').matches ? 'auto' : 'smooth' });
+
   const handleTap = (n) => {
     const sel = window.getSelection();
     if (sel && !sel.isCollapsed && sel.toString().trim()) return; // user is selecting text, not tapping
@@ -92,6 +103,23 @@ export default function ReaderText({
 
       {!loading && !error && sideBySide && !parallel.error && (
         <div style={bodyStyle} className="font-serif">
+          {diff && (
+            <div className="mb-4 rounded-xl border border-[var(--r-line)] bg-[var(--r-bar)] px-4 py-3 font-sans text-sm" style={{ fontSize: '0.875rem' }}>
+              <p>
+                <strong>{parallel.translation}</strong>{diff.similarity === 1 ? ' uses the same words as ' : <> is <strong>{differenceLabel(diff.similarity)}</strong> from </>}{translationId} in this chapter
+                {diff.total > 0 && <span className="text-[var(--r-sub)]"> · {diff.differing} of {diff.total} verses differ</span>}
+              </p>
+              {diff.mostDifferent.length > 0 && (
+                <p className="mt-1 text-xs text-[var(--r-sub)]">
+                  Most different:{' '}
+                  {diff.mostDifferent.map((v, i) => (
+                    <span key={v.number}>{i > 0 && ', '}<button type="button" onClick={() => jumpToVerse(v.number)} className="font-medium text-[var(--r-accent)] hover:underline">verse {v.number}</button> ({Math.round((1 - v.similarity) * 100)}%)</span>
+                  ))}
+                </p>
+              )}
+              <p className="mt-1 text-xs text-[var(--r-sub)]"><span className="diff-hl">Marked words</span> aren’t in the other wording.</p>
+            </div>
+          )}
           <div className="mb-3 hidden grid-cols-2 gap-x-10 border-b border-[var(--r-line)] pb-1 font-sans text-[11px] uppercase tracking-wide text-[var(--r-sub)] md:grid">
             <span>{translationId}</span><span>{parallel.translation}</span>
           </div>
@@ -99,12 +127,14 @@ export default function ReaderText({
             <div key={row.number} className="mb-4 grid gap-x-10 gap-y-1 md:grid-cols-2">
               <p>
                 {row.first
-                  ? <span {...verseProps(row.first)}>{num(row.first.number)}{renderVerseText(row.first.number, row.first.text, searchLower)}{chips(row.first)}</span>
+                  ? <span {...verseProps(row.first)}>{num(row.first.number)}{renderVerseText(row.first.number, row.first.text, searchLower, diffByVerse.get(row.number)?.a)}{chips(row.first)}</span>
                   : <span className="text-[var(--r-sub)]">{num(row.number)}—</span>}
               </p>
               <p className="border-l-2 border-[var(--r-line)] pl-3 text-[0.95em] text-[var(--r-sub)] md:border-l-0 md:pl-0 md:text-[var(--r-text)]">
                 <span className="mr-1.5 font-sans text-[10px] uppercase tracking-wide md:hidden">{parallel.translation}</span>
-                {row.second ? <>{num(row.second.number)}{row.second.text}</> : parallel.loading ? <span className="font-sans text-xs" aria-label="Loading">…</span> : '—'}
+                {row.second ? <>{num(row.second.number)}<DiffText text={row.second.text} ranges={diffByVerse.get(row.number)?.b} />{diffByVerse.get(row.number) && !diffByVerse.get(row.number).same && (
+                  <span className="ml-2 whitespace-nowrap font-sans text-[10px] text-[var(--r-sub)]">{differenceLabel(diffByVerse.get(row.number).similarity)}</span>
+                )}</> : parallel.loading ? <span className="font-sans text-xs" aria-label="Loading">…</span> : '—'}
               </p>
             </div>
           ))}