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/components/OfflineSave.jsx

@@ -0,0 +1,32 @@
+// The "Save for offline" control: a button, or progress with a Stop button while a download runs.
+import { canSaveOffline } from '../utils/offline.js';
+
+export function DownloadProgress({ job, onCancel }) {
+  const pct = job.total ? Math.round((job.done / job.total) * 100) : 0;
+  return (
+    <div role="status" className="rounded-xl border border-[var(--r-line)] px-3 py-2 text-xs">
+      <div className="mb-1.5 flex items-center justify-between gap-2">
+        <span>Saving {job.label}… {job.done} of {job.total}</span>
+        <button type="button" onClick={onCancel} className="text-[var(--r-accent)] hover:underline">Stop</button>
+      </div>
+      <div className="h-1 overflow-hidden rounded-full bg-[var(--r-line)]"><div className="h-full bg-[var(--r-accent)] transition-[width]" style={{ width: `${pct}%` }} /></div>
+    </div>
+  );
+}
+
+// One book of a translation.
+export function SaveBookOffline({ offline, translation, book, name }) {
+  if (!canSaveOffline(translation)) {
+    return <p className="mt-4 text-[11px] text-[var(--r-sub)]">Saving for offline works with the BSB and NET. The {translation}’s licence doesn’t allow it.</p>;
+  }
+  if (offline.job) return <div className="mt-4"><DownloadProgress job={offline.job} onCancel={offline.cancel} /></div>;
+  const saved = offline.isSaved(translation, book);
+  return (
+    <div className="mt-4 flex items-center justify-between gap-3 rounded-xl border border-[var(--r-line)] px-3 py-2 text-sm">
+      <span className={saved ? '' : 'text-[var(--r-sub)]'}>{saved ? `✓ ${name} (${translation}) is saved for offline` : `Read ${name} without a connection`}</span>
+      {!saved && (
+        <button type="button" onClick={() => offline.saveBook(translation, book, name)} className="shrink-0 rounded-full border border-[var(--r-line)] px-3 py-1 text-xs font-medium hover:bg-black/5">Save for offline</button>
+      )}
+    </div>
+  );
+}