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/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} />;
 }