Set initial chapter to first downloaded chapter if there is no history when starting to read a local comic. Close #405

lib/foundation/local.dart

@@ -109,15 +109,42 @@ class LocalComic with HistoryMixin implements Comic {
 
   void read() {
     var history = HistoryManager().find(id, comicType);
+    int? firstDownloadedChapter;
+    int? firstDownloadedChapterGroup;
+    if (downloadedChapters.isNotEmpty && chapters != null) {
+      final chapters = this.chapters!;
+      if (chapters.isGrouped) {
+        for (int i=0; i<chapters.groupCount; i++) {
+          var group = chapters.getGroupByIndex(i);
+          var keys = group.keys.toList();
+          for (int j=0; j<keys.length; j++) {
+            var chapterId = keys[j];
+            if (downloadedChapters.contains(chapterId)) {
+              firstDownloadedChapter = j + 1;
+              firstDownloadedChapterGroup = i + 1;
+              break;
+            }
+          }
+        }
+      } else {
+        var keys = chapters.allChapters.keys;
+        for (int i = 0; i < keys.length; i++) {
+          if (downloadedChapters.contains(keys.elementAt(i))) {
+            firstDownloadedChapter = i + 1;
+            break;
+          }
+        }
+      }
+    }
     App.rootContext.to(
       () => Reader(
         type: comicType,
         cid: id,
         name: title,
         chapters: chapters,
-        initialChapter: history?.ep,
+        initialChapter: history?.ep ?? firstDownloadedChapter,
         initialPage: history?.page,
-        initialChapterGroup: history?.group,
+        initialChapterGroup: history?.group ?? firstDownloadedChapterGroup,
         history: history ??
             History.fromModel(
               model: this,