refactor: split MainViewModel core under 500 lines — 500-limit Phase 3, Commit A

Core MainViewModel.cs 758 -> 495 lines. Moved the source add/remove/image
block (AddSource, NextSourceName, AddImage, BuildImageCandidates,
PickImageBytes, AddAsset, AddImageSource, AddReusedImage, RemoveElement) into a
new MainViewModel.Sources.cs partial, and WebView2 source hosting
(RegisterLoadedWebSources, InitWebView2, OnWebView2PreviewBitmapChanged) into a
new MainViewModel.Web.cs partial. Pruned now-dead usings from core.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 3 tracker updated in same commit.

ViewModels/MainViewModel.Web.cs

@@ -0,0 +1,41 @@
+using System.Linq;
+using System.Windows.Controls;
+using System.Windows.Media.Imaging;
+using ytLive.Models;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.ViewModels;
+
+public partial class MainViewModel
+{
+    private void RegisterLoadedWebSources()
+    {
+        if (_webView2Manager == null) return;
+
+        foreach (var source in Scenes.SelectMany(s => s.Elements).OfType<Source>()
+                     .Where(s => s.Type == SourceType.WebSource))
+        {
+            _webView2Manager.Register(source);
+            source.PropertyChanged += (_, e) =>
+            {
+                if (e.PropertyName == nameof(Source.WebUri))
+                    _webView2Manager.OnWebUriChanged(source);
+            };
+        }
+    }
+
+    public void InitWebView2(Panel hostPanel)
+    {
+        _webView2Manager = new WebView2Manager(hostPanel, System.Windows.Application.Current.Dispatcher);
+        _webView2Manager.PreviewBitmapChanged += OnWebView2PreviewBitmapChanged;
+        RegisterLoadedWebSources();
+    }
+
+    private void OnWebView2PreviewBitmapChanged(string sourceId, WriteableBitmap bitmap)
+    {
+        foreach (var scene in Scenes)
+            foreach (var el in scene.Elements.OfType<Source>().Where(s => s.Id == sourceId))
+                el.VideoImageSource = bitmap;
+    }
+}