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/index.css

@@ -49,3 +49,36 @@ select {
 .scrollbar-thin::-webkit-scrollbar-track {
   background: rgba(148, 163, 184, 0.12);
 }
+
+/* A clear ring wherever keyboard focus lands (mouse clicks don't show it). */
+:where(button, a, summary, [role='button'], [role='tab'], [role='option'], input, select, textarea):focus-visible {
+  outline: 2px solid var(--r-accent, #a84e27);
+  outline-offset: 2px;
+}
+
+/* "Skip to content" appears only when it's tabbed to. */
+.skip-link {
+  position: absolute;
+  left: 0.75rem;
+  top: -4rem;
+  z-index: 100;
+  padding: 0.5rem 0.9rem;
+  border-radius: 9999px;
+  background: var(--r-text, #2b2620);
+  color: var(--r-bg, #faf7f0);
+  font-size: 0.875rem;
+}
+.skip-link:focus {
+  top: 0.75rem;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  *,
+  *::before,
+  *::after {
+    animation-duration: 0.01ms !important;
+    animation-iteration-count: 1 !important;
+    scroll-behavior: auto !important;
+    transition-duration: 0.01ms !important;
+  }
+}