feat: marketplace GPU spec cards with VRAM, CUDA, region, pricing

- Add GPU fields to OfferRecord type (gpu_model, gpu_memory_gb, etc.)
- Replace flat table with responsive GPU offer cards
- Show GPU model, VRAM, GPU count, CUDA version, capacity specs
- Display pricing and SLA with provider + region
- Add offer-card CSS with spec grid, hover effects, responsive layout

apps/marketplace-web/src/lib/api.ts

@@ -9,6 +9,13 @@ interface OfferRecord {
   price: number;
   sla: string;
   status: string;
+  created_at?: string;
+  gpu_model?: string;
+  gpu_memory_gb?: number;
+  gpu_count?: number;
+  cuda_version?: string;
+  price_per_hour?: number;
+  region?: string;
 }
 
 interface OffersResponse {