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/editor/positions.ts

@@ -0,0 +1,27 @@
+export interface ChapterPosition {
+  from: number;
+  to: number;
+  scroll: number;
+}
+
+const KEY = "margin-positions";
+
+type Store = Record<string, Record<string, ChapterPosition>>;
+
+function readAll(): Store {
+  try {
+    return JSON.parse(localStorage.getItem(KEY) || "{}") as Store;
+  } catch {
+    return {};
+  }
+}
+
+export function loadPosition(bookId: string, chapterId: string): ChapterPosition | null {
+  return readAll()[bookId]?.[chapterId] ?? null;
+}
+
+export function savePosition(bookId: string, chapterId: string, position: ChapterPosition): void {
+  const all = readAll();
+  (all[bookId] ||= {})[chapterId] = position;
+  localStorage.setItem(KEY, JSON.stringify(all));
+}