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/utils/outlineLabels.js

@@ -0,0 +1,28 @@
+// How outline points are numbered. Kept free of imports so both the page and the exporters can use it.
+
+const ROMAN = [[10, 'X'], [9, 'IX'], [5, 'V'], [4, 'IV'], [1, 'I']];
+export function toRoman(n) {
+  let out = '';
+  let rest = n;
+  for (const [value, symbol] of ROMAN) while (rest >= value) { out += symbol; rest -= value; }
+  return out;
+}
+const letter = (n, upper) => {
+  let out = '';
+  let rest = n;
+  while (rest > 0) { rest -= 1; out = String.fromCharCode((upper ? 65 : 97) + (rest % 26)) + out; rest = Math.floor(rest / 26); }
+  return out;
+};
+
+// I. / A. / 1. / a. / (1) / (a), by level
+export function labelFor(depth, index) {
+  const n = index + 1;
+  switch (depth) {
+    case 0: return `${toRoman(n)}.`;
+    case 1: return `${letter(n, true)}.`;
+    case 2: return `${n}.`;
+    case 3: return `${letter(n, false)}.`;
+    case 4: return `(${n})`;
+    default: return `(${letter(n, false)})`;
+  }
+}