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-tauri/capabilities/default.json

@@ -5,6 +5,7 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
+    "core:window:allow-destroy",
     "opener:default",
     "dialog:default",
     "updater:default",

src/App.tsx

@@ -1,10 +1,11 @@
 import { useEffect } from "react";
 import { listen } from "@tauri-apps/api/event";
+import { getCurrentWindow } from "@tauri-apps/api/window";
 import { Library } from "./components/Library";
 import { EditorView } from "./components/EditorView";
 import { useBook } from "./store/useBook";
 import { isDesktop } from "./ipc";
-import { newBook } from "./library";
+import { newBook, saveBook } from "./library";
 import { runExport } from "./export/run";
 import { checkForUpdates } from "./updater";
 
@@ -27,6 +28,21 @@ function App() {
     };
   }, []);
 
+  useEffect(() => {
+    if (!isDesktop) return;
+    const win = getCurrentWindow();
+    const unlisten = win.onCloseRequested(async (event) => {
+      const { book, dirty } = useBook.getState();
+      if (!book || !dirty) return;
+      event.preventDefault();
+      await saveBook(book).catch(() => {});
+      win.destroy();
+    });
+    return () => {
+      unlisten.then((stop) => stop());
+    };
+  }, []);
+
   return book ? <EditorView /> : <Library onOpen={openBook} />;
 }
 

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) =>