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

website/docs/css/docs.css

@@ -15,9 +15,9 @@
 
 /* Dark mode (default) */
 :root {
-    --primary-color: #3b82f6;
-    --secondary-color: #2563eb;
-    --accent-color: #60a5fa;
+    --primary-color: #2563eb;
+    --secondary-color: #7c3aed;
+    --accent-color: #0ea5e9;
     --success-color: #10b981;
     --warning-color: #f59e0b;
     --danger-color: #ef4444;
@@ -33,8 +33,8 @@
 /* Light mode */
 body.light {
     --primary-color: #2563eb;
-    --secondary-color: #1e40af;
-    --accent-color: #3b82f6;
+    --secondary-color: #7c3aed;
+    --accent-color: #0ea5e9;
     --success-color: #10b981;
     --warning-color: #f59e0b;
     --danger-color: #ef4444;