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/hooks/usePrefsSync.js

@@ -5,7 +5,7 @@
 import { useCallback, useEffect, useRef } from 'react';
 import { getRemotePrefs, saveRemotePrefs } from '../syncService.js';
 
-export const PREF_KEYS = ['theme', 'translation', 'layout', 'commentarySource', 'planTranslation', 'startPage', 'position'];
+export const PREF_KEYS = ['theme', 'translation', 'layout', 'commentarySource', 'planTranslation', 'startPage', 'position', 'colorLabels'];
 const META_KEY = 'prefs-meta'; // { key: timestamp of the last change made on this device }
 const PUSH_DELAY_MS = 1000;
 const FOCUS_PULL_MIN_MS = 30_000;
@@ -58,7 +58,7 @@ export function usePrefsSync({ values, apply, canApplyPosition = () => true }) {
       window.clearTimeout(timerRef.current);
       timerRef.current = window.setTimeout(push, PUSH_DELAY_MS);
     }
-  }, [values.theme, values.translation, values.layout, values.commentarySource, values.planTranslation, values.startPage, values.position.book, values.position.chapter, push]); // eslint-disable-line react-hooks/exhaustive-deps
+  }, [values.theme, values.translation, values.layout, values.commentarySource, values.planTranslation, values.startPage, values.position.book, values.position.chapter, values.colorLabels, push]); // eslint-disable-line react-hooks/exhaustive-deps
 
   const pullServerPrefs = useCallback(async () => {
     enabledRef.current = true;
@@ -69,6 +69,7 @@ export function usePrefsSync({ values, apply, canApplyPosition = () => true }) {
     const meta = readMeta();
     const now = Date.now();
     for (const key of PREF_KEYS) {
+      if (valuesRef.current[key] === undefined) continue; // nothing on this device to compare or upload
       const remote = server[key];
       const localAt = meta[key] ?? 0;
       if (remote && remote.at > localAt) {