fix: move gesture-handler and TrackPlayer init to index.js, fix Dimensions usage

mobile/App.js

@@ -1,4 +1,3 @@
-import 'react-native-gesture-handler';
 import React, { useEffect, useState } from 'react';
 import { View, StatusBar } from 'react-native';
 import { NavigationContainer } from '@react-navigation/native';
@@ -8,7 +7,6 @@ import { SafeAreaProvider } from 'react-native-safe-area-context';
 import { GestureHandlerRootView } from 'react-native-gesture-handler';
 import { Ionicons } from '@expo/vector-icons';
 import TrackPlayer, { Event, State } from 'react-native-track-player';
-import { PlaybackService } from './src/audio/service';
 import { setupPlayer } from './src/audio/player';
 import { useStore } from './src/store';
 
@@ -21,8 +19,6 @@ import LoginScreen        from './src/screens/Login';
 import NowPlayingScreen   from './src/screens/NowPlaying';
 import PlayerBar          from './src/components/PlayerBar';
 
-TrackPlayer.registerPlaybackService(() => PlaybackService);
-
 const Tab   = createBottomTabNavigator();
 const Stack = createStackNavigator();