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/DiffText.jsx

@@ -0,0 +1,13 @@
+// Text with the words that differ from the other translation marked.
+export function DiffText({ text, ranges = [] }) {
+  if (!ranges.length) return text;
+  const parts = [];
+  let cursor = 0;
+  ranges.forEach((r, i) => {
+    if (r.start > cursor) parts.push(text.slice(cursor, r.start));
+    parts.push(<span key={i} className="diff-hl">{text.slice(r.start, r.end)}</span>);
+    cursor = r.end;
+  });
+  if (cursor < text.length) parts.push(text.slice(cursor));
+  return <>{parts}</>;
+}