fix(editor): persist per-chapter cursor and scroll across restarts

Positions were kept in an in-memory Map, so they were lost on quit and
the cursor jumped to the top on reopen. Store them in localStorage
(keyed by book + chapter, like theme/width) and restore on app open,
not just on in-session chapter switches. Also save the current chapter
continuously (debounced on selection + scroll) so closing the app
records where you left off, and re-apply scroll after fonts load.

src/components/EditorView.tsx

@@ -263,6 +263,7 @@ export function EditorView() {
                   )}
                 </header>
                 <Editor
+                  bookId={book.id}
                   chapterId={chapter.id}
                   content={chapter.content}
                   onChange={(content) => setChapterContent(chapter.id, content)}