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/projectSync.js

@@ -0,0 +1,18 @@
+// Stops one device from silently overwriting a study another device has saved since. The browser sends
+// the `lastEdited` of the server copy it last saw (X-Base-Edited); if the server's copy has moved past
+// that, the save is refused with 409 and the person chooses whose version to keep.
+
+// stored: the server copy's lastEdited (null when there is none); base: what the client last saw
+// (null/undefined when it never synced this study); force: the person chose "keep mine".
+export function isSaveConflict({ stored, base, force = false }) {
+  if (force || stored == null) return false;
+  if (!Number.isFinite(base)) return false; // a device that never saw the server's copy has nothing to compare
+  return stored > base;
+}
+
+// Reads the base header; anything that isn't a plain number is treated as "unknown".
+export function parseBase(header) {
+  if (header == null || header === '') return null;
+  const n = Number(header);
+  return Number.isFinite(n) ? n : null;
+}