fix: replace onCloseRequested with beforeunload to fix window close

The Tauri onCloseRequested async handler was preventing the window
from actually closing. Using beforeunload for the save flush instead.

src/App.tsx

@@ -1,5 +1,4 @@
 import { useState, useEffect, useCallback } from "react";
-import { getCurrentWindow } from "@tauri-apps/api/window";
 import { AnimatePresence, motion } from "framer-motion";
 import { springs, fadeSlideLeft, fadeSlideRight } from "@/lib/motion";
 import { useAppStore } from "@/stores/app-store";
@@ -27,11 +26,12 @@ export default function App() {
 
   // Flush pending board saves before the app window closes
   useEffect(() => {
-    const unlisten = getCurrentWindow().onCloseRequested(async () => {
+    function handleBeforeUnload() {
       useBoardStore.getState().closeBoard();
-    });
+    }
+    window.addEventListener("beforeunload", handleBeforeUnload);
     return () => {
-      unlisten.then((fn) => fn());
+      window.removeEventListener("beforeunload", handleBeforeUnload);
     };
   }, []);