Web source: off-screen WebView2 capture + layout persistence + transparent bg

Bundle exception (2026-08-27): this lands prior-session work plus today's
transparency fix as one commit per creator instruction.

WebView2Manager (new): off-screen WebView2 control parked at -5000/-5000 in
the window's visual tree, captures via CapturePreviewAsync every 100 ms into
a BGRA8 VideoFrame. Source Width/Height sync to the control via
PropertyChanged. JS transparent-background injection runs on every
NavigationStarting/NavigationCompleted so the page's own CSS does not paint
white over the capture.

Transparency fix: DefaultBackgroundColor = System.Drawing.Color.Transparent
set on the WPF control before EnsureCoreWebView2Async. The WPF SDK doc says
the value is forwarded to the controller on init; the IDL (ICoreWebView2-
Controller2) confirms alpha=0 makes the capture alpha-preserving. The
prior-session 'CoreWebView2.BackgroundColor' path was the wrong property
name and has been replaced.

LayoutStore: WebUri column added (idempotent ALTER TABLE) and round-tripped
on save/load (schema v11).

MainViewModel: WebView2Manager constructed on Loaded; loaded web sources
re-registered; Source.WebUri PropertyChanged forwards to manager.

MainWindow: off-screen host Canvas added to visual tree.

Tests: 3 scale-sync + WebView2_DefaultBackgroundColor_Is_Transparent +
LayoutStorePersistenceTests.WebSource_WebUri_Persists. 234/236 pass
(2 pre-existing failures unchanged).

Scale bug (DPI/physical-pixel mismatch) still open.

MainWindow.xaml.cs

@@ -27,6 +27,7 @@ public partial class MainWindow : Window
         InitializeComponent();
         AppLog.Write("MainWindow ctor: after InitializeComponent");
         _viewModel = (MainViewModel)DataContext;
+        _viewModel.InitWebView2(WebViewHostPanel);
         _viewModel.PropertyChanged += OnViewModelPropertyChanged;
         _viewModel.ChatMessages.CollectionChanged += OnChatMessagesChanged;
         UpdateTaskbarOverlay();