feat: add skeleton loaders to marketplace and integrate global header across docs

Marketplace:
- Add skeleton loading states for stats grid and GPU offer cards
- Show animated skeleton placeholders during data fetch
- Add skeleton CSS with shimmer animation and dark mode support
- Wrap stats section in #stats-grid container for skeleton injection

Trade Exchange:
- Replace inline header with data-global-header component
- Switch GPU offers to production API (/api/miners/list)
- Add fallback to demo

apps/marketplace-web/src/style.css

@@ -8,6 +8,42 @@
   -moz-osx-font-smoothing: grayscale;
 }
 
+/* Skeleton loading styles */
+.skeleton {
+  background: linear-gradient(90deg, #f0f0f0 25%, #e0e0e0 50%, #f0f0f0 75%);
+  background-size: 200% 100%;
+  animation: skeleton-loading 1.5s infinite;
+  border-radius: 4px;
+}
+
+@keyframes skeleton-loading {
+  0% { background-position: 200% 0; }
+  100% { background-position: -200% 0; }
+}
+
+.skeleton-card {
+  height: 120px;
+  margin-bottom: 1rem;
+}
+
+.skeleton-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
+  gap: 1rem;
+  margin-bottom: 2rem;
+}
+
+.skeleton-list {
+  display: flex;
+  flex-direction: column;
+  gap: 1rem;
+}
+
+.dark .skeleton {
+  background: linear-gradient(90deg, #374151 25%, #4b5563 50%, #374151 75%);
+  background-size: 200% 100;
+}
+
 /* Dark mode variables */
 .dark {
   --bg-primary: #1f2937;