implement avg buildtime field in header

frontend/lib/models/build.dart

@@ -4,7 +4,8 @@ class Build {
   final int pkg_id;
   final String version;
   final int status;
-  final int? start_time, end_time;
+  final DateTime? end_time;
+  final DateTime start_time;
 
   Build(
       {required this.id,
@@ -16,12 +17,18 @@ class Build {
       required this.status});
 
   factory Build.fromJson(Map<String, dynamic> json) {
+    final startTime =
+        DateTime.fromMillisecondsSinceEpoch(json["start_time"] * 1000);
+    final endTime = json["end_time"] != null
+        ? DateTime.fromMillisecondsSinceEpoch((json["end_time"] as int) * 1000)
+        : null;
+
     return Build(
       id: json["id"] as int,
       pkg_id: json["pkg_id"] as int,
       status: json["status"] as int,
-      start_time: json["start_time"] as int?,
-      end_time: json["end_time"] as int?,
+      start_time: startTime,
+      end_time: endTime,
       pkg_name: json["pkg_name"] as String,
       version: json["version"] as String,
     );

frontend/lib/models/stats.dart

@@ -2,17 +2,17 @@ class Stats {
   final int total_builds,
       failed_builds,
       avg_queue_wait_time,
-      avg_build_time,
       repo_storage_size,
       active_builds,
       total_packages;
+  final Duration avg_build_time;
 
   factory Stats.fromJson(Map<String, dynamic> json) {
     return Stats(
       total_builds: json["total_builds"] as int,
       failed_builds: json["failed_builds"] as int,
       avg_queue_wait_time: json["avg_queue_wait_time"] as int,
-      avg_build_time: json["avg_build_time"] as int,
+      avg_build_time: Duration(seconds: json["avg_build_time"]),
       repo_storage_size: json["repo_storage_size"] as int,
       active_builds: json["active_builds"] as int,
       total_packages: json["total_packages"] as int,