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/useOfflineSave.js

@@ -0,0 +1,52 @@
+// Drives "Save for offline": one download at a time, with progress for the button to show,
+// a cancel, and a toast when it finishes.
+import { useCallback, useEffect, useRef, useState } from 'react';
+import { useToast } from '../components/Toasts.jsx';
+import { bibleRefs, bookRefs, downloadChapters, loadSavedBooks, markBookSaved } from '../utils/offline.js';
+
+export function useOnline() {
+  const [online, setOnline] = useState(() => (typeof navigator === 'undefined' ? true : navigator.onLine !== false));
+  useEffect(() => {
+    const up = () => setOnline(true);
+    const down = () => setOnline(false);
+    window.addEventListener('online', up);
+    window.addEventListener('offline', down);
+    return () => { window.removeEventListener('online', up); window.removeEventListener('offline', down); };
+  }, []);
+  return online;
+}
+
+export function useOfflineSave() {
+  const { toast } = useToast();
+  const [saved, setSaved] = useState(loadSavedBooks);
+  const [job, setJob] = useState(null); // { label, done, total } while downloading
+  const controller = useRef(null);
+  useEffect(() => () => controller.current?.abort(), []);
+
+  const run = useCallback(async (label, translation, refs, { bookKeys = [] } = {}) => {
+    if (controller.current) return; // one at a time
+    const abort = new AbortController();
+    controller.current = abort;
+    setJob({ label, done: 0, total: refs.length });
+    const result = await downloadChapters(translation, refs, { signal: abort.signal, onProgress: (done, total) => setJob({ label, done, total }) });
+    controller.current = null;
+    setJob(null);
+    if (abort.signal.aborted) { toast('Download stopped'); return; }
+    if (result.failed === 0) {
+      bookKeys.forEach((b) => markBookSaved(translation, b));
+      setSaved(loadSavedBooks());
+      toast(`Saved ${label} for offline`);
+    } else {
+      toast(`Saved ${result.done} of ${refs.length} chapters — check your connection and try again`, { tone: 'error' });
+    }
+  }, [toast]);
+
+  return {
+    saved, job,
+    isSaved: (translation, book) => (saved[translation] ?? []).includes(book),
+    saveBook: (translation, book, name) => run(name, translation, bookRefs(book), { bookKeys: [book] }),
+    saveBible: (translation) => run(`The whole ${translation}`, translation, bibleRefs(), { bookKeys: [...new Set(bibleRefs().map((r) => r.book))] }),
+    saveRefs: (label, translation, refs) => run(label, translation, refs),
+    cancel: () => controller.current?.abort(),
+  };
+}