fix(persist): flush pending save on navigate-away and quit

Edits are saved on an 800ms debounce, so leaving a book within that
window lost the latest changes. Flush a dirty book before openBook /
closeBook swap state (covers All books, open-another, Cmd+N), and add a
window close-request handler that saves before destroying on quit.

Claude-Session: https://claude.ai/code/session_01RzTWrLiy4zGuw2hZSQGamk

src/store/useBook.ts

@@ -13,6 +13,7 @@ import {
   normalizeBook,
 } from "../model/book";
 import { loadActiveChapter, saveActiveChapter } from "../editor/positions";
+import { saveBook } from "../library";
 
 export const COVER_ID = "__cover__";
 
@@ -41,7 +42,11 @@ interface BookState {
   markSaved: () => void;
 }
 
-export const useBook = create<BookState>((set) => ({
+function flushOutgoing(book: Book | null, dirty: boolean): void {
+  if (book && dirty) saveBook(book).catch(() => {});
+}
+
+export const useBook = create<BookState>((set, get) => ({
   book: null,
   activeChapterId: "",
   dirty: false,
@@ -50,13 +55,17 @@ export const useBook = create<BookState>((set) => ({
   setExporting: (label) => set({ exporting: label }),
   setNotice: (message) => set({ notice: message }),
   openBook: (book) => {
+    flushOutgoing(get().book, get().dirty);
     const normalized = normalizeBook(book);
     const saved = loadActiveChapter(normalized.id);
     const valid = saved === COVER_ID || normalized.chapters.some((c) => c.id === saved);
     const activeChapterId = valid ? (saved as string) : normalized.chapters[0]?.id ?? "";
     set({ book: normalized, activeChapterId, dirty: false });
   },
-  closeBook: () => set({ book: null, activeChapterId: "", dirty: false }),
+  closeBook: () => {
+    flushOutgoing(get().book, get().dirty);
+    set({ book: null, activeChapterId: "", dirty: false });
+  },
   setActiveChapter: (id) => set({ activeChapterId: id }),
   setChapterContent: (id, content) =>
     set((s) =>