home page: avoid refetching sheets on rebuild

lib/home_page.dart

@@ -25,11 +25,13 @@ class _MyHomePageState extends State<MyHomePage> {
   String? appName;
   String? appVersion;
   bool shuffling = false;
+  late final Future<List<Sheet>> sheets;
 
   @override
   void initState() {
     super.initState();
     _loadAppInfo();
+    sheets = acquireSheets();
   }
 
   Future<void> _loadAppInfo() async {
@@ -148,7 +150,7 @@ class _MyHomePageState extends State<MyHomePage> {
       appBar: AppBar(title: const Text("Sheetless")),
       endDrawer: _buildDrawer(),
       body: FutureBuilder(
-        future: acquireSheets(),
+        future: sheets,
         builder: (BuildContext context, AsyncSnapshot<List<Sheet>> snapshot) {
           if (snapshot.hasData) {
             return SheetsWidget(