design: wire the sans family into the window; fonts ship as explicit Resource items

The bundled faces were packaged but unused — no style referenced FontFamily.Sans, so the app
still rendered Segoe UI. MainWindow now sets FontFamily from the token, which cascades to every
child (dialogs follow in the sweep).

Font packaging: six explicit <Resource> items instead of the Assets\Fonts\*.ttf glob, so the
guard can name exactly what must ship.

Investigation worth recording: a runtime assertion that the families resolve from
pack://application:,,,/ would have needed an Application instance inside the test host, and
without one WPF resolves the pack authority against the test host — every probe failed,
including the app icon that demonstrably works. The resources ARE embedded (the lowercase
`assets/fonts/inter-regular.ttf` entry is present in ytLive.g.resources); the assertion was
removed rather than faked, and the real confirmation is a launch, which is a visual check.

Guard remains structural: files are real binaries, each is a Resource in the csproj, the theme
uses pack URIs, notices + both OFL texts are present, and no third family sneaks in.

ytLive.Tests/BundledFontsTests.cs

@@ -1,5 +1,10 @@
+using System;
 using System.IO;
+using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
+using System.Windows;
+using System.Windows.Media;
 using Xunit;
 
 namespace ytLive.Tests;
@@ -30,8 +35,10 @@ public class BundledFontsTests
             Assert.True(new FileInfo(path).Length > 50_000, $"{name} looks like a placeholder");
         }
 
-        // 2. The project packages them as WPF resources.
-        Assert.Contains(@"Assets\Fonts\*.ttf", SourceAssert.Text("ytLive.csproj"));
+        // 2. The project packages each one as a WPF resource (an explicit item, not a guess).
+        var csproj = SourceAssert.Text("ytLive.csproj");
+        foreach (var name in Files)
+            Assert.Contains($@"Assets\Fonts\{name}", csproj);
 
         // 3. The theme resolves each family from the pack folder — a bare family name would
         //    only work if the user happened to have the font installed.