feat: show snackbar on playback speed change

close #824

lib/services/keyboard_shortcuts_service.dart

@@ -8,6 +8,7 @@ import '../i18n/strings.g.dart';
 import '../mpv/mpv.dart';
 import 'settings_service.dart';
 import '../utils/player_utils.dart';
+import '../utils/snackbar_helper.dart';
 
 class KeyboardShortcutsService {
   static KeyboardShortcutsService? _instance;
@@ -293,15 +294,18 @@ class KeyboardShortcutsService {
         final newRateUp = (player.state.rate + 0.25).clamp(0.25, 3.0);
         player.setRate(newRateUp);
         _settingsService.setDefaultPlaybackSpeed(newRateUp);
+        showGlobalSnackBar(_formatSpeed(newRateUp));
         break;
       case 'speed_decrease':
         final newRateDown = (player.state.rate - 0.25).clamp(0.25, 3.0);
         player.setRate(newRateDown);
         _settingsService.setDefaultPlaybackSpeed(newRateDown);
+        showGlobalSnackBar(_formatSpeed(newRateDown));
         break;
       case 'speed_reset':
         player.setRate(1.0);
         _settingsService.setDefaultPlaybackSpeed(1.0);
+        showGlobalSnackBar(_formatSpeed(1.0));
         break;
       case 'sub_seek_next':
         player.command(['sub-seek', '1']);
@@ -387,4 +391,9 @@ class KeyboardShortcutsService {
 
     return aModifiers.length == bModifiers.length && aModifiers.every((modifier) => bModifiers.contains(modifier));
   }
+
+  String _formatSpeed(double speed) {
+    final s = speed.toStringAsFixed(2).replaceFirst(RegExp(r'\.?0+$'), '');
+    return '${s}x';
+  }
 }

lib/utils/snackbar_helper.dart

@@ -61,6 +61,13 @@ void showGlobalErrorSnackBar(String message) {
   );
 }
 
+/// Shows an info snackbar using the root ScaffoldMessenger (survives navigation).
+void showGlobalSnackBar(String message, {Duration duration = const Duration(seconds: 2)}) {
+  rootScaffoldMessengerKey.currentState
+    ?..removeCurrentSnackBar()
+    ..showSnackBar(SnackBar(content: Text(message), duration: duration));
+}
+
 /// Shows a success snackbar with a message
 ///
 /// [context] The build context