feat: Multi Window Support

[0xApp]

Multi Window Support

Summary

• Add IWindowManager service for opening and managing multiple desktop windows, each with an independent Blazor component tree
• Add Razor component for declarative window management (open on render, close on dispose)
• Refactor BlazorDesktopWindow to accept WindowOptions POCO instead of reading IConfiguration directly, enabling child windows with custom settings
• Add multi-window demo page to BlazorDesktop.Sample showcasing both service-based and component-based APIs
• Update README with multi-window documentation, API reference, and updated examples

New files

• Hosting/WindowOptions.cs - Per-window configuration POCO with FromConfiguration() factory
• Hosting/DesktopWindowHandle.cs - Lightweight handle to a managed window (Id, IsMainWindow, Closed event)
• Services/IWindowManager.cs - Public interface: OpenAsync(), CloseAsync(), MainWindow, events
• Services/WindowManager.cs - Internal implementation with ConcurrentDictionary tracking, WPF Dispatcher marshaling
• Components/DesktopWindow.cs - Blazor component wrapping IWindowManager for declarative usage

Modified files

• Wpf/BlazorDesktopWindow.cs - Added internal constructor accepting WindowOptions + RootComponentMappingCollection; replaced all IConfiguration reads with _options field
• Hosting/BlazorDesktopHostBuilder.cs - Replaced BlazorDesktopWindow singleton with IWindowManager/WindowManager singleton
• Services/BlazorDesktopService.cs - Creates BlazorDesktopWindow manually on STA thread; registers with WindowManager

Breaking change

BlazorDesktopWindow is no longer registered in the DI container. Code that injected it directly must use IWindowManager.MainWindow.NativeWindow instead.

Test plan

• dotnet build BlazorDesktop.sln succeeds with 0 warnings/errors
• Run BlazorDesktop.Sample, verify main window works identically to before
• Navigate to Multi-Window page, click "Open Window (Service)" - child window opens with correct title/size and independent counter
• Click "Open Window (Component)" - toggle opens/closes a declarative child window
• Close a child window via X button - Closed event fires, window count updates, main app stays open
• Close main window - all child windows close and app exits

[russkyc]

This is how i would imagine #42 would be api-wise, @andrewbabbittdev what do you think?

[andrewbabbittdev]

Mmmm generally prefer to start with a design proposal before starting on a PR @russkyc I would still like to see one filled out to go with this PR. It is under the issue templates.

I just got back from vacation so I wont have time to go over this for a little as I catch up on work and other stuff.