Refactor and enhance Download Manager application

- Added new services: IContentDialogService, ISnackbarService
- Replaced DashboardPage and DashboardViewModel with DownloadingPage, DownloadingPageViewModel, WaitingPage, and WaitingPageViewModel
- Introduced AddNewTaskPage and AddNewTaskViewModel
- Replaced DownloadServiceManger with DownloadManagerService
- Updated WPF-UI package to 3.0.4 and added HtmlAgilityPack 1.11.61
- Refactored DownloadWorker to use async methods and added retry logic
- Converted DownloadItemData to partial class with ObservableProperty attributes
- Changed initial navigation page to DownloadingPage
- Updated MainWindow navigation items and title
- Added new utility classes: DoubleUtilities, MD5Utilities, StringExtension
- Added new XAML pages and ViewModels for task management and display
- Removed obsolete DashboardPage and related files

Models/UrlGetter/IUrlGetter.cs

@@ -0,0 +1,12 @@
+namespace DownloadManager.Models.UrlGetter;
+public struct FileUrl(string? url, string? name, string? md5)
+{
+    public string? Url { get; } = url;
+    public string? Name { get; } = name;
+    public string? MD5 { get; } = md5;
+}
+
+public interface IUrlGetter
+{
+    public Task<FileUrl> GetFileUrl(string taskUrl);
+}