feat(welcome): Hybrid glass-card shell in MainLayout

windows/welcome/src/SilverOS.Welcome.App/Components/Layout/MainLayout.razor

@@ -1,17 +1,8 @@
 @inherits LayoutComponentBase
 
-<div class="page">
-    <div class="sidebar">
-        <NavMenu />
+<div class="sm-wall"></div>
+<div class="sm-glass">
+    <div class="sm-body">
+        @Body
     </div>
-
-    <main>
-        <div class="top-row px-4">
-            <a href="https://learn.microsoft.com/aspnet/core/" target="_blank">About</a>
-        </div>
-
-        <article class="content px-4">
-            @Body
-        </article>
-    </main>
 </div>