Add symbol for offline available sheets

lib/core/models/sheet.dart

@@ -7,12 +7,14 @@ class Sheet {
   String name;
   String composer;
   DateTime updatedAt;
+  bool availableOffline;
 
   Sheet({
     required this.uuid,
     required this.name,
     required this.composer,
     required this.updatedAt,
+    this.availableOffline = false,
   });
 
   /// Creates a [Sheet] from a JSON map returned by the API.

lib/core/services/api_client.dart

@@ -254,6 +254,13 @@ class ApiClient {
     return cachedFile;
   }
 
+  Future<bool> isPdfFileCached(String sheetUuid) async {
+    final cacheDir = await getTemporaryDirectory();
+    final cachedFile = File('${cacheDir.path}/$sheetUuid.pdf');
+
+    return await cachedFile.exists();
+  }
+
   // ---------------------------------------------------------------------------
   // Annotation Operations
   // ---------------------------------------------------------------------------