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/reader/readerThemes.test.js

@@ -0,0 +1,27 @@
+import { describe, expect, test } from 'vitest';
+import { READER_THEMES } from './readerThemes.js';
+
+// WCAG relative luminance and contrast ratio
+const luminance = (hex) => {
+  const [r, g, b] = [1, 3, 5].map((i) => parseInt(hex.slice(i, i + 2), 16) / 255)
+    .map((c) => (c <= 0.03928 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4));
+  return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+};
+const contrast = (a, b) => {
+  const [hi, lo] = [luminance(a), luminance(b)].sort((x, y) => y - x);
+  return (hi + 0.05) / (lo + 0.05);
+};
+
+// Small text needs 4.5:1. These are the pairings the app actually draws.
+describe.each(Object.entries(READER_THEMES))('%s theme contrast', (_id, { vars }) => {
+  const v = (name) => vars[`--r-${name}`];
+  test.each([
+    ['text', 'bg'], ['text', 'bar'], ['text', 'hl'],
+    ['sub', 'bg'], ['sub', 'bar'], ['sub', 'hl'],
+    ['accent', 'bg'], ['accent', 'bar'],
+    ['danger', 'bg'], ['danger', 'bar'],
+    ['on', 'accent'],
+  ])('%s on %s is at least 4.5:1', (fg, bg) => {
+    expect(contrast(v(fg), v(bg))).toBeGreaterThanOrEqual(4.5);
+  });
+});