Add QoL feed, download and quality improvements

lib/backend/models/download_task.dart

@@ -0,0 +1,46 @@
+import 'post.dart';
+
+enum DownloadTaskStatus {
+  queued,
+  running,
+  completed,
+  failed,
+  canceled,
+}
+
+class DownloadTask {
+  const DownloadTask({
+    required this.id,
+    required this.post,
+    required this.fileName,
+    required this.progress,
+    required this.status,
+    this.savedPath,
+    this.error,
+  });
+
+  final String id;
+  final Post post;
+  final String fileName;
+  final double progress;
+  final DownloadTaskStatus status;
+  final String? savedPath;
+  final String? error;
+
+  DownloadTask copyWith({
+    double? progress,
+    DownloadTaskStatus? status,
+    String? savedPath,
+    String? error,
+  }) {
+    return DownloadTask(
+      id: id,
+      post: post,
+      fileName: fileName,
+      progress: progress ?? this.progress,
+      status: status ?? this.status,
+      savedPath: savedPath ?? this.savedPath,
+      error: error,
+    );
+  }
+}