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/useHistoryNav.js

@@ -0,0 +1,42 @@
+// Makes the browser's (and phone's) Back/Forward buttons move between the app's pages. The app keeps
+// its page in state rather than in the URL, so without this Back would leave the app altogether.
+// Each page change adds a history entry; Back/Forward set the page again from that entry.
+import { useEffect, useRef } from 'react';
+
+// Pages that only make sense with a study project open.
+export const NEEDS_PROJECT = ['study', 'outline', 'script', 'export'];
+
+export function useHistoryNav({ page, setPage, hasProject, onArrive, enabled = true }) {
+  const shown = useRef(page);
+  const hasProjectRef = useRef(hasProject);
+  hasProjectRef.current = hasProject;
+  const onArriveRef = useRef(onArrive);
+  onArriveRef.current = onArrive;
+
+  // First entry: record where the app started so Back can return to it.
+  useEffect(() => {
+    if (!enabled) return;
+    try { window.history.replaceState({ page }, ''); } catch { /* history unavailable */ }
+  }, []); // eslint-disable-line react-hooks/exhaustive-deps
+
+  // A page change made inside the app adds a history entry.
+  useEffect(() => {
+    if (!enabled || shown.current === page) return;
+    shown.current = page;
+    try { window.history.pushState({ page }, ''); } catch { /* history unavailable */ }
+  }, [page, enabled]);
+
+  useEffect(() => {
+    if (!enabled) return undefined;
+    const onPop = (e) => {
+      let target = e.state?.page;
+      if (!target) return; // an entry that isn't ours (e.g. a hash change)
+      if (NEEDS_PROJECT.includes(target) && !hasProjectRef.current) target = 'home';
+      shown.current = target; // so the effect above doesn't push this page again
+      setPage(target);
+      onArriveRef.current?.(target);
+    };
+    window.addEventListener('popstate', onPop);
+    return () => window.removeEventListener('popstate', onPop);
+  }, [enabled, setPage]);
+}