Add max-width constraint to main content area

Keep sidebar full-height at left edge while constraining the main
content area to max-w-7xl centered.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/publisher-dashboard/src/lib/components/layout/dashboard-layout.svelte

@@ -22,8 +22,10 @@ let { title, children, class: className }: Props = $props();
   <div class="flex flex-1 flex-col overflow-hidden">
     <AppHeader {title} />
 
-    <main class={cn("flex-1 overflow-auto p-4 lg:p-6", className)}>
+    <main class="flex-1 overflow-auto p-4 lg:p-6">
+      <div class={cn("mx-auto max-w-7xl", className)}>
         {@render children()}
+      </div>
     </main>
   </div>
 </div>