feat: add window close handler, configure minimum window size

Flush pending board saves on window close via Tauri's onCloseRequested.
Set minimum window dimensions to 800x600.

src/App.tsx

@@ -1,5 +1,7 @@
 import { useState, useEffect, useCallback } from "react";
+import { getCurrentWindow } from "@tauri-apps/api/window";
 import { useAppStore } from "@/stores/app-store";
+import { useBoardStore } from "@/stores/board-store";
 import { AppShell } from "@/components/layout/AppShell";
 import { BoardList } from "@/components/boards/BoardList";
 import { BoardView } from "@/components/board/BoardView";
@@ -18,6 +20,16 @@ export default function App() {
     init();
   }, [init]);
 
+  // Flush pending board saves before the app window closes
+  useEffect(() => {
+    const unlisten = getCurrentWindow().onCloseRequested(async () => {
+      useBoardStore.getState().closeBoard();
+    });
+    return () => {
+      unlisten.then((fn) => fn());
+    };
+  }, []);
+
   // Listen for custom event to open settings from TopBar or command palette
   useEffect(() => {
     function handleOpenSettings() {