feat(player): support playback speeds up to 8x

close #1545

lib/media/playback_rate.dart

@@ -0,0 +1,3 @@
+/// Inclusive playback-rate bounds supported by every in-app player backend.
+const double minimumPlaybackRate = 0.25;
+const double maximumPlaybackRate = 8.0;

lib/mpv/player/player.dart

@@ -1,6 +1,7 @@
 import 'dart:io' show Platform;
 
 import '../../media/media_display_criteria.dart';
+import '../../media/playback_rate.dart';
 import '../models.dart';
 import 'platform/player_android.dart';
 import 'player_native.dart';
@@ -159,7 +160,7 @@ abstract class Player {
 
   /// Set the playback rate/speed.
   ///
-  /// [rate] - Playback rate from 0.25 to 4.0 (1.0 = normal speed).
+  /// [rate] - Playback rate from [minimumPlaybackRate] to [maximumPlaybackRate] (1.0 = normal speed).
   Future<void> setRate(double rate);
 
   /// Set the audio output device.

lib/services/keyboard_shortcuts_service.dart

@@ -3,6 +3,7 @@ import 'dart:io';
 
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import '../media/playback_rate.dart';
 import '../models/hotkey_model.dart';
 import '../i18n/strings.g.dart';
 import '../mpv/mpv.dart';
@@ -412,12 +413,12 @@ class KeyboardShortcutsService extends ChangeNotifier {
         onPreviousEpisode?.call();
         break;
       case 'speed_increase':
-        final newRateUp = (player.state.rate + 0.25).clamp(0.25, 3.0);
+        final newRateUp = (player.state.rate + 0.25).clamp(minimumPlaybackRate, maximumPlaybackRate);
         player.setRate(newRateUp);
         _settingsService.write(SettingsService.defaultPlaybackSpeed, newRateUp);
         break;
       case 'speed_decrease':
-        final newRateDown = (player.state.rate - 0.25).clamp(0.25, 3.0);
+        final newRateDown = (player.state.rate - 0.25).clamp(minimumPlaybackRate, maximumPlaybackRate);
         player.setRate(newRateDown);
         _settingsService.write(SettingsService.defaultPlaybackSpeed, newRateDown);
         break;

lib/services/settings_service.dart

@@ -1,5 +1,6 @@
 import 'dart:convert';
 import '../media/ids.dart';
+import '../media/playback_rate.dart';
 import '../media/media_version_preference.dart';
 import 'dart:io';
 import 'package:flutter/foundation.dart';
@@ -476,7 +477,7 @@ class SettingsService extends BaseSharedPreferencesService {
   static final defaultPlaybackSpeed = DoublePref(
     'default_playback_speed',
     defaultValue: 1.0,
-    transform: (v) => v.clamp(0.5, 3.0),
+    transform: (v) => v.clamp(minimumPlaybackRate, maximumPlaybackRate),
   );
   static final defaultBoxFitMode = IntPref('default_box_fit_mode', transform: (v) => v.clamp(0, 2));
   static final displaySwitchDelay = IntPref('display_switch_delay', transform: (v) => v.clamp(0, 10));

lib/watch_together/services/host_playback_coordinator.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 import 'dart:math';
 
+import '../../media/playback_rate.dart';
 import '../../utils/app_logger.dart';
 import '../models/playback_state.dart';
 import '../models/watch_session.dart';
@@ -60,8 +61,8 @@ class HostPlaybackCoordinator {
   static const int seekDebounceMs = 200;
   static const int implicitJumpThresholdMs = 1500;
   static const int selfRecoveryMinBufferAheadMs = 2000;
-  static const double _minimumRemoteRate = 0.25;
-  static const double _maximumRemoteRate = 4.0;
+  static const double _minimumRemoteRate = minimumPlaybackRate;
+  static const double _maximumRemoteRate = maximumPlaybackRate;
 
   final String myPeerId;
   final void Function(PlaybackState state, {String? toPeerId}) _sendState;

lib/widgets/video_controls/sheets/video_settings_sheet.dart

@@ -13,6 +13,7 @@ import 'package:provider/provider.dart';
 
 import '../../../models/shader_preset.dart';
 import '../../../models/transcode_quality_preset.dart';
+import '../../../media/playback_rate.dart';
 import '../../../media/media_version.dart';
 import '../../../mpv/mpv.dart';
 import '../../../providers/shader_provider.dart';
@@ -765,7 +766,26 @@ class _VideoSettingsSheetState extends State<VideoSettingsSheet> {
       initialData: widget.player.state.rate,
       builder: (context, snapshot) {
         final currentRate = snapshot.data ?? 1.0;
-        final speeds = [0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75, 3.0];
+        const speeds = <double>[
+          0.5,
+          0.75,
+          1.0,
+          1.25,
+          1.5,
+          1.75,
+          2.0,
+          2.25,
+          2.5,
+          2.75,
+          3.0,
+          3.5,
+          4.0,
+          4.5,
+          5.0,
+          6.0,
+          7.0,
+          maximumPlaybackRate,
+        ];
 
         return ListView.builder(
           itemCount: speeds.length,