update dark mode handling

frontend/lib/main.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:frontend/provider/theme_provider.dart';
 import 'package:go_router/go_router.dart';
 import 'package:provider/provider.dart';
 
@@ -17,10 +18,17 @@ import 'pages/leaderboard.dart';
 
 void main() {
   runApp(
-    ChangeNotifierProvider(
-      create: (_) => QuoteProvider(),
-      child: HlaskovnikFrontendApp(),
-    ),
+    MultiProvider(
+      providers: [
+        ChangeNotifierProvider(
+          create: (_) => QuoteProvider()
+        ),
+        ChangeNotifierProvider(
+          create: (_) => ThemeProvider()
+        )
+      ],
+      child: HlaskovnikFrontendApp()
+    )
   );
 }
 
@@ -46,6 +54,7 @@ class HlaskovnikFrontendApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final themeProvider = context.watch<ThemeProvider>();
     return MaterialApp.router(
       title: Strings.appName,
       theme: ThemeData(
@@ -62,7 +71,7 @@ class HlaskovnikFrontendApp extends StatelessWidget {
         ),
         useMaterial3: true,
       ),
-      themeMode: ThemeMode.system,
+      themeMode: themeProvider.isDark ? ThemeMode.dark : ThemeMode.system,
       routerConfig: _router,
     );
   }

frontend/lib/pages/list.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import 'package:frontend/provider/quote_provider.dart';
 import 'package:frontend/model/quote.dart';
+import 'package:frontend/provider/theme_provider.dart';
 import 'package:frontend/values/strings.dart';
 import 'package:provider/provider.dart';
 
@@ -13,15 +14,16 @@ class ListPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final provider = context.watch<QuoteProvider>();
+    final themeProvider = context.watch<ThemeProvider>();
     return RefreshIndicator(
       onRefresh: () async {
         await provider.readQuotes();
       },
-      child: _bodyBuilder(context, provider),
+      child: _bodyBuilder(context, provider, themeProvider),
     );
   }
 
-  Widget _bodyBuilder(BuildContext context, QuoteProvider provider) {   
+  Widget _bodyBuilder(BuildContext context, QuoteProvider provider, ThemeProvider themeProvider) {   
     if (provider.loading) {
       return Center(child: CircularProgressIndicator());
     }
@@ -51,7 +53,7 @@ class ListPage extends StatelessWidget {
               ),
             ),
           ),
-          filterBar(context, provider),
+          filterBar(context, provider, themeProvider),
           Expanded(
             child: ListView.builder(
               padding: EdgeInsets.all(Constants.edgeInset),
@@ -220,7 +222,7 @@ class ListPage extends StatelessWidget {
     );
   }
 
-  Widget filterBar(BuildContext context, QuoteProvider provider) {
+  Widget filterBar(BuildContext context, QuoteProvider provider, ThemeProvider themeProvider) {
     return Padding(
       padding: const EdgeInsets.all(Constants.edgeInset),
       child: Row(
@@ -246,7 +248,7 @@ class ListPage extends StatelessWidget {
           IconButton(
             icon: Icon(Icons.more_horiz),
             onPressed: () {
-              _showOptionsBottomSheet(context, provider);
+              _showOptionsBottomSheet(context, provider, themeProvider);
             },
           ),
         ],
@@ -254,7 +256,7 @@ class ListPage extends StatelessWidget {
     );
   }
 
-  void _showOptionsBottomSheet(BuildContext context, QuoteProvider provider) {
+  void _showOptionsBottomSheet(BuildContext context, QuoteProvider provider, ThemeProvider themeProvider) {
     showModalBottomSheet(
       context: context,
       builder: (context) {
@@ -284,6 +286,25 @@ class ListPage extends StatelessWidget {
                     );
                   }
                 ),
+                ListTile(
+                  title: Text(Strings.userPrefersDarkMode),
+                  onTap: () { themeProvider.setDarkMode(!themeProvider.isDark); },
+                  trailing: Row(
+                    mainAxisSize: MainAxisSize.min,
+                    children: [
+                      Padding(
+                        padding: const EdgeInsets.only(right: Constants.edgeInset),
+                        child: Switch(
+                          onChanged: (value) {
+                            themeProvider.setDarkMode(value);
+                          }, 
+                          value: themeProvider.isDark
+                        ),
+                      ),
+                      Icon(Icons.dark_mode),
+                    ],
+                  ),
+                ),
               ],
             ),
         );

frontend/lib/provider/theme_provider.dart

@@ -0,0 +1,34 @@
+
+import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+Future<void> saveDarkMode(bool isDark) async {
+  final prefs = await SharedPreferences.getInstance();
+  await prefs.setBool('darkMode', isDark);
+}
+
+Future<bool> loadDarkMode() async {
+  final prefs = await SharedPreferences.getInstance();
+  return prefs.getBool('darkMode') ?? false; // default light
+}
+
+class ThemeProvider extends ChangeNotifier {
+  bool _isDark = false;
+
+  bool get isDark => _isDark;
+
+  ThemeProvider() {
+    _load(); // load saved value on startup
+  }
+
+  void _load() async {
+    _isDark = await loadDarkMode();
+    notifyListeners();
+  }
+
+  void setDarkMode(bool value) async {
+    _isDark = value;
+    notifyListeners();
+    await saveDarkMode(value);
+  }
+}

frontend/lib/values/strings.dart

@@ -30,4 +30,6 @@ class Strings {
   static const String filterBy = "Filtrovat podle";
   static const String exportSelectedQuotes = "Exportovat vybrané hlášky";
   static const String byAttribute = "podle příznaku";
+
+  static const String userPrefersDarkMode = "Preferuji tmavý režim";
 }