add author selection menu

frontend/lib/provider/quote_provider.dart

@@ -7,10 +7,12 @@ import 'package:frontend/repository/quote_repository.dart';
 
 class QuoteProvider extends ChangeNotifier {
   List<Quote> _quotes = [];
+  Set<String> _authors = {};
   bool _loading = false;
   String? _error;
 
   List<Quote> get quotes => _quotes;
+  Set<String> get authors => _authors;
   bool get loading => _loading;
   String? get error => _error;
 
@@ -20,6 +22,13 @@ class QuoteProvider extends ChangeNotifier {
     readQuotes(); // auto-load on creation
   }
 
+  void populateAuthors() {
+    _authors = {};
+    for (var i = 0; i < quotes.length; i++) {
+      _authors.add(quotes[i].author);
+    }
+  }
+
   Future<void> readQuotes() async {
     _loading = true;
     _error = null;
@@ -31,6 +40,8 @@ class QuoteProvider extends ChangeNotifier {
       _error = e.toString();
     }
 
+    populateAuthors();
+
     _loading = false;
     notifyListeners();
   }
@@ -38,6 +49,7 @@ class QuoteProvider extends ChangeNotifier {
   Future<void> createQuote(Quote quote) async {
     final newQuote = await repo.createQuote(quote);
     _quotes.add(newQuote);
+    populateAuthors();
     notifyListeners();
   }
 
@@ -50,11 +62,20 @@ class QuoteProvider extends ChangeNotifier {
 
       final index = _quotes.indexWhere((q) => q.id == quote.id);
       if (index != -1) {
+        final oldAuthor = _quotes[index].author;
         final newList = List<Quote>.from(_quotes);
         newList[index] = updated;
         _quotes = newList;
+        Map<String, int> authorMap = {};
+        for (final q in quotes) {
+          authorMap[q.author] = (authorMap[q.author] ?? 0) + 1;
+        }
+        if (authorMap[oldAuthor] == 1) {
+          _authors.remove(oldAuthor);
+        }
+        _authors.add(updated.author);
       }
-    } finally {
+    } finally {      
       _loading = false;
       notifyListeners();
     }
@@ -64,6 +85,7 @@ class QuoteProvider extends ChangeNotifier {
     await repo.deleteQuote(quote);
 
     _quotes.removeWhere((q) => q.id == quote.id);
+    populateAuthors();
     notifyListeners();
   }
 }