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.csproj

@@ -81,7 +81,12 @@
     <Resource Include="Assets\chat-background.jpg" />
     <Resource Include="Assets\alert-default.mp4" />
     <!-- Design-standard typefaces (DESIGN.md): Inter + JetBrains Mono, OFL-1.1. -->
-    <Resource Include="Assets\Fonts\*.ttf" />
+    <Resource Include="Assets\Fonts\Inter-Regular.ttf" />
+    <Resource Include="Assets\Fonts\Inter-Medium.ttf" />
+    <Resource Include="Assets\Fonts\Inter-SemiBold.ttf" />
+    <Resource Include="Assets\Fonts\Inter-Bold.ttf" />
+    <Resource Include="Assets\Fonts\JetBrainsMono-Medium.ttf" />
+    <Resource Include="Assets\Fonts\JetBrainsMono-SemiBold.ttf" />
   </ItemGroup>
 
   <ItemGroup>