STYLED - Roll out the layered card system to the admin workspace

The double-layer AppCard look showcased in the UI Kit was only applied on
the public frontend; every admin surface was pinned to layer=single and
stayed flat. Convert the admin section/container cards to the default
layered look, keeping compact metric tiles, toolbars, image tiles and
nested leaf cards single.

Add AppCard contentClassName so content utilities (space-y-*, grid, flex)
keep applying to the inner shell after the switch to double layer.

components/admin/smtp-settings-form.tsx

@@ -25,7 +25,7 @@ export function SMTPSettingsForm({
   return (
     <form id="smtp-settings-form" action={action} className="space-y-6">
       <div className="grid gap-6 xl:grid-cols-2">
-        <AppCard layer="single">
+        <AppCard>
           <CardHeader>
             <CardTitle>SMTP Connection</CardTitle>
             <CardDescription>Server und Login.</CardDescription>
@@ -109,7 +109,7 @@ export function SMTPSettingsForm({
           </CardContent>
         </AppCard>
 
-        <AppCard layer="single">
+        <AppCard>
           <CardHeader>
             <CardTitle>Sender And Recipients</CardTitle>
             <CardDescription>Absender und Ziele.</CardDescription>