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/utils/highlightColors.js

@@ -0,0 +1,16 @@
+// The colours a highlight can be, and the meanings readers tend to give them. The reader names
+// each colour themselves (see the Highlights list); these are only the starting suggestions.
+export const HIGHLIGHT_COLORS = ['#fef08a', '#bbf7d0', '#bfdbfe', '#fecaca', '#e9d5ff', '#fed7aa'];
+
+export const COLOR_NAMES = {
+  '#fef08a': 'Yellow', '#bbf7d0': 'Green', '#bfdbfe': 'Blue',
+  '#fecaca': 'Red', '#e9d5ff': 'Purple', '#fed7aa': 'Orange',
+};
+
+export const COLOR_SUGGESTIONS = {
+  '#fef08a': 'Key verse', '#bbf7d0': 'Promise', '#bfdbfe': 'Command',
+  '#fecaca': 'Warning', '#e9d5ff': 'Prayer', '#fed7aa': 'Question',
+};
+
+// "Promise (green)" when named, otherwise just "Green".
+export const colorTitle = (color, labels = {}) => (labels[color] ? `${labels[color]} (${(COLOR_NAMES[color] ?? 'color').toLowerCase()})` : COLOR_NAMES[color] ?? 'Highlight');