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/prefs.js

@@ -10,10 +10,20 @@ const ENUMS = {
   commentarySource: ['matthew-henry', 'john-gill', 'jamieson-fausset-brown', 'tyndale', 'adam-clarke', 'john-calvin', 'keil-delitzsch'],
 };
 const DAY_MS = 86_400_000;
+const COLOR = /^#[0-9a-fA-F]{3,8}$/;
 
 // A cleaned value, or undefined when it isn't acceptable for that preference.
 export function cleanPrefValue(key, value) {
   if (ENUMS[key]) return ENUMS[key].includes(value) ? value : undefined;
+  if (key === 'colorLabels') {
+    // What each highlight colour means to the reader: { "#fef08a": "Promises", ... }
+    if (!value || typeof value !== 'object' || Array.isArray(value)) return undefined;
+    const out = {};
+    for (const [color, label] of Object.entries(value).slice(0, 12)) {
+      if (COLOR.test(color) && typeof label === 'string' && label.trim()) out[color] = label.trim().slice(0, 30);
+    }
+    return out;
+  }
   if (key === 'position') {
     if (!value || typeof value !== 'object') return undefined;
     const book = String(value.book ?? '').toUpperCase();