add authorCount variable for mapping how many quotes everybody has

frontend/lib/provider/quote_provider.dart

@@ -38,6 +38,20 @@ class QuoteProvider extends ChangeNotifier {
     return filtered;
   }
 
+  Map<String, int> get authorCounts {
+    final counts = <String, int>{};
+
+    for (var quote in filteredQuotes) {
+      counts.update(
+        quote.author,
+        (value) => value + 1,
+        ifAbsent: () => 1,
+      );
+    }
+
+    return counts;
+  }
+
   final QuoteRepository repo = QuoteRepository();
 
   int filterBy = 0; // 0 = by author, 1 = by quote