Add Outline stage, parallel reading, offline saves, backup, sync-conflict handling and more

Study
- New Outline stage (hierarchical I./A./1. points, linked to chunks, in all exports);
  the chunk stage is now "Chunks"
- Home cards: passages shown as ranges, tags capped with "+N more"

Reader
- Side-by-side translations and a Compare tab per verse
- Typeface, line spacing, verse-number settings; text and column scale with the screen
- Highlights/bookmarks sheet with named colours (synced); BSB footnote spacing fix
- Cmd-K palette (passages, studies, notes, marks), keyboard shortcuts and "?" help

Flow and safety
- Toasts with Undo replace most confirm()/alert()
- Back/Forward move between pages
- Server refuses saves over a newer copy (409); keep-mine / use-other banner;
  opening a study no longer re-stamps it
- Latest chapter request wins when flipping quickly

Offline and data
- Save books, plan days or a whole BSB/NET for offline
- Service worker fixes: helloao /api/ URLs were skipped and responses cloned too late
- Settings: download/restore a full backup

Accessibility
- Sheets trap focus and close on Escape, skip link, focus ring, reduced motion,
  theme contrast raised to 4.5:1 (with a test)

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

src/hooks/useParallelChapter.js

@@ -0,0 +1,25 @@
+// The second Bible in the side-by-side view: loads the same chapter in another translation.
+// Pass an empty translation to switch it off.
+import { useEffect, useState } from 'react';
+import { fetchChapter } from '../utils/bibleText.js';
+
+export function useParallelChapter(translation, bookAbbrev, chapter) {
+  const [state, setState] = useState({ key: '', verses: [], error: '' });
+  const key = translation ? `${translation}/${bookAbbrev}/${chapter}` : '';
+
+  useEffect(() => {
+    if (!key) return undefined;
+    const controller = new AbortController();
+    fetchChapter(translation, bookAbbrev, chapter, { signal: controller.signal })
+      .then(({ verses }) => setState({ key, verses, error: '' }))
+      .catch((err) => { if (!controller.signal.aborted) setState({ key, verses: [], error: err.message || 'Unable to load.' }); });
+    return () => controller.abort();
+  }, [key]); // eslint-disable-line react-hooks/exhaustive-deps
+
+  const ready = state.key === key;
+  return {
+    verses: ready ? state.verses : [],
+    error: ready ? state.error : '',
+    loading: !!key && !ready,
+  };
+}