Add symbol for offline available sheets

lib/features/home/home_page.dart

@@ -125,6 +125,10 @@ class _HomePageState extends State<HomePage> with RouteAware {
     _sheets = await _sortSheetsByRecency(result.sheets);
     _isOnline = result.isOnline;
 
+    for (var sheet in _sheets) {
+      sheet.availableOffline = await _apiClient!.isPdfFileCached(sheet.uuid);
+    }
+
     return result;
   }
 

lib/features/home/widgets/sheet_list_item.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import 'package:flutter/foundation.dart';
 import '../../../core/models/sheet.dart';
 
 /// A list tile displaying a single sheet's information.
@@ -24,6 +25,10 @@ class SheetListItem extends StatelessWidget {
       subtitle: Text(sheet.composer),
       onTap: onTap,
       onLongPress: onLongPress,
+      trailing: Visibility(
+        visible: !kIsWeb && sheet.availableOffline,
+        child: Icon(Icons.download_done, color: Colors.green),
+      ),
     );
   }
 }

lib/features/sheet_viewer/sheet_viewer_page.dart

@@ -98,6 +98,7 @@ class _SheetViewerPageState extends State<SheetViewerPage>
       // Native: use file cache
       final file = await widget.apiClient.getPdfFileCached(widget.sheet.uuid);
       _document = await PdfDocument.openFile(file.path);
+      widget.sheet.availableOffline = true;
     }
 
     setState(() {