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/useChunkBuilder.js

@@ -2,6 +2,7 @@
 // active chapter tab, and every way of creating chunks (click, shift-click,
 // typed range, bulk ranges, cross-chapter spans), plus reorder and delete.
 import { useEffect, useState } from 'react';
+import { useToast } from '../components/Toasts.jsx';
 import { bookOptions, CHAPTER_COUNTS, makeId, parseBibleChapter } from '../utils/bibleUtils.jsx';
 
 // Every chunk starts blank; all note fields exist from the start.
@@ -99,6 +100,7 @@ function initialChapterIndex(project) {
 }
 
 export function useChunkBuilder({ project, setProject, updateProject, allChunks, setCurrentPage }) {
+  const { toast } = useToast();
   const [setup, setSetup] = useState(() => initialSetupForm(project));
   const [titleEdited, setTitleEdited] = useState(false);
   const [activeChapterIndex, setActiveChapterIndex] = useState(() => initialChapterIndex(project));
@@ -531,7 +533,33 @@ export function useChunkBuilder({ project, setProject, updateProject, allChunks,
     }));
   };
 
+  // Deleting a chunk also deletes its notes, so it can be taken back for a few seconds.
   const deleteChunk = (chunkId) => {
+    let removed = null;
+    (project?.chapters ?? []).forEach((ch, chapterIndex) => {
+      const index = ch.chunks.findIndex((c) => c.id === chunkId);
+      if (index >= 0) removed = { chunk: ch.chunks[index], chapterIndex, index, wasSelected: project.selectedChunkId === chunkId };
+    });
+    if (removed) {
+      toast('Chunk deleted', {
+        action: {
+          label: 'Undo',
+          onClick: () => updateProject((current) => {
+            if (current.chapters.some((ch) => ch.chunks.some((c) => c.id === chunkId))) return current;
+            return {
+              ...current,
+              selectedChunkId: removed.wasSelected ? chunkId : current.selectedChunkId,
+              chapters: current.chapters.map((ch, i) => {
+                if (i !== removed.chapterIndex) return ch;
+                const chunks = [...ch.chunks];
+                chunks.splice(Math.min(removed.index, chunks.length), 0, removed.chunk);
+                return { ...ch, chunks };
+              }),
+            };
+          }),
+        },
+      });
+    }
     updateProject((current) => {
       const newAllChunks = current.chapters
         .flatMap((ch) => ch.chunks)