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>

server/index.js

@@ -6,7 +6,7 @@ import { randomUUID } from 'crypto';
 import { fileURLToPath } from 'url';
 import { dirname, join } from 'path';
 import {
-  initDb, getAllProjects, getProject, upsertProject, deleteProject,
+  initDb, getAllProjects, getProject, getProjectEdited, upsertProject, deleteProject,
   countUsers, createUser, getUserByEmail, getUserById, claimOrphanProjects,
   enableTotp, disableTotp, setBackupCodeHashes, setPodcastName,
   getShareToken, setShareToken, clearShareToken, getProjectByShareToken,
@@ -26,6 +26,7 @@ import { startReminderScheduler, sendTestReminder, isValidTimeZone } from './rem
 import { registerBibleRoutes } from './bibleApi.js';
 import { cleanMarks } from './readerMarks.js';
 import { cleanPrefsPatch, mergePrefs } from './prefs.js';
+import { isSaveConflict, parseBase } from './projectSync.js';
 
 // Shape returned to the client for the signed-in user (login, MFA verify, /auth/me).
 function userPayload(user) {
@@ -398,6 +399,11 @@ app.put('/api/projects/:id', requireAuth, (req, res) => {
     }
     if (body.id.length > 100) return res.status(400).json({ error: 'Invalid project id.' });
     if (body.title.length > 500) return res.status(400).json({ error: 'Project title is too long.' });
+    // Refuse to overwrite a copy another device saved after the one this device last saw
+    const stored = getProjectEdited(body.id, req.session.userId);
+    if (isSaveConflict({ stored, base: parseBase(req.get('X-Base-Edited')), force: req.query.force === '1' })) {
+      return res.status(409).json({ error: 'This study was changed on another device.', code: 'conflict', serverEdited: stored });
+    }
     const saved = upsertProject(body, req.session.userId);
     if (!saved) {
       return res.status(403).json({ error: 'That project belongs to a different account.' });