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/syncBase.js

@@ -0,0 +1,9 @@
+// Remembers, per study, the server copy's lastEdited that this device last saw — the "base" a save is
+// checked against so another device's newer copy is never overwritten silently. Kept in memory: it is
+// rebuilt from the server's list each time you sign in.
+const base = new Map();
+
+export const getSyncBase = (id) => base.get(id);
+export const setSyncBase = (id, lastEdited) => { if (Number.isFinite(lastEdited)) base.set(id, lastEdited); };
+export const clearSyncBase = (id) => base.delete(id);
+export const resetSyncBases = () => base.clear();