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/index.css

@@ -82,3 +82,10 @@ select {
     transition-duration: 0.01ms !important;
   }
 }
+
+/* Words that differ between two translations (Reader → read beside another Bible) */
+.diff-hl {
+  background: color-mix(in srgb, var(--r-accent, #a84e27) 20%, transparent);
+  border-bottom: 2px solid color-mix(in srgb, var(--r-accent, #a84e27) 55%, transparent);
+  border-radius: 2px;
+}