Support local file paths for M3U playlists; UI tweaks

Added support for loading M3U playlists from both web URLs and local file paths, with logic to resolve relative paths. Improved exception handling in M3UParser. Enabled toggling of window border style in fullscreen mode, with data binding for WindowStyle. Removed IsValidUrl check to allow local file sources in settings. Cleaned up usings and formatting. Removed .NET SDK version from global.json.

TV Player WPF/ViewModels/MainViewModel.cs

@@ -36,6 +36,14 @@ namespace TV_Player
             get => _currentWindowState;
             set => SetProperty(ref _currentWindowState, value);
         }
+
+        private WindowStyle _currentWindowStyle;
+        public WindowStyle CurrentWindowStyle
+        {
+            get => _currentWindowStyle;
+            set => SetProperty(ref _currentWindowStyle, value);
+        }
+
         public ICommand OnKeyDownCommand { get; }
 
         public ICommand FullscreenCommand { get; }
@@ -53,6 +61,7 @@ namespace TV_Player
             FullscreenCommand = new RelayCommand(OnFullSctreenButtonClick);
             SettingsCommand = new RelayCommand(OnSettingsButtonClick);
             CloseAppCommand = new RelayCommand(OnCloseAppButtonClick);
+            CurrentWindowStyle = WindowStyle.SingleBorderWindow;
         }
 
         public void OnFullSctreenButtonClick()
@@ -60,10 +69,13 @@ namespace TV_Player
             if (CurrentWindowState == WindowState.Normal)
             {
                 CurrentWindowState = WindowState.Maximized;
+                CurrentWindowStyle = WindowStyle.None;
+
             }
             else
             {
                 CurrentWindowState = WindowState.Normal;
+                CurrentWindowStyle = WindowStyle.SingleBorderWindow;
             }
         }