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/syncService.js

@@ -10,18 +10,19 @@
 
 const BASE = '/api';
 
-async function request(method, path, body) {
+async function request(method, path, body, { keepalive = false, headers = {} } = {}) {
   try {
     const opts = {
       method,
-      headers: { 'Content-Type': 'application/json' },
+      headers: { 'Content-Type': 'application/json', ...headers },
       credentials: 'include',
     };
+    if (keepalive) opts.keepalive = true; // lets the request finish even as the tab closes
     if (body !== undefined) opts.body = JSON.stringify(body);
     const res = await fetch(`${BASE}${path}`, opts);
     const data = await res.json().catch(() => null);
     if (!res.ok) {
-      return { ok: false, status: res.status, error: data?.error ?? `HTTP ${res.status}` };
+      return { ok: false, status: res.status, error: data?.error ?? `HTTP ${res.status}`, data };
     }
     return { ok: true, data };
   } catch (err) {
@@ -51,10 +52,14 @@ export async function loadRemoteProject(id) {
 
 /**
  * Save (create or update) a project on the server.
+ * `baseEdited` is the server copy's lastEdited this device last saw: if the server has moved on, the
+ * save is refused with { ok: false, status: 409, data: { serverEdited } }. `force` overwrites anyway.
  * Returns { ok, data: summary }
  */
-export async function saveRemoteProject(project) {
-  return request('PUT', `/projects/${project.id}`, project);
+export async function saveRemoteProject(project, { baseEdited, force = false } = {}) {
+  return request('PUT', `/projects/${project.id}${force ? '?force=1' : ''}`, project, {
+    headers: Number.isFinite(baseEdited) ? { 'X-Base-Edited': String(baseEdited) } : {},
+  });
 }
 
 /**
@@ -62,7 +67,7 @@ export async function saveRemoteProject(project) {
  * Returns { ok }
  */
 export async function deleteRemoteProject(id) {
-  return request('DELETE', `/projects/${id}`);
+  return request('DELETE', `/projects/${id}`, undefined, { keepalive: true });
 }
 
 // ---------------------------------------------------------------------------