Fix price display issues and layout in design review page

- Add defensive checks in cart store to handle missing/NaN unitPrice values
- Display '—' for items with invalid prices instead of NaN
- Fix design review page grid layout to align buttons at top of page
- Ensure buttons don't get pushed below images on review page

src/app/designs/[id]/review/page.tsx

@@ -124,7 +124,7 @@ export default function DesignReviewPage({ params }: { params: { id: string } })
       <h1 className="font-display text-3xl mb-2">{design.product.name}</h1>
       <p className="text-muted mb-8">Review your personalized design</p>
 
-      <div className="grid gap-8 md:grid-cols-2">
+      <div className="grid gap-8 md:grid-cols-2 md:items-start">
         {/* Design Preview */}
         <div className="space-y-4">
           <div>