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/assets/js/skeleton.js

@@ -0,0 +1,139 @@
+/**
+ * Skeleton Loading States for AITBC Website
+ * Provides loading placeholders while content is being fetched
+ */
+
+class SkeletonLoader {
+    constructor() {
+        this.skeletonStyles = `
+            .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-text {
+                height: 1rem;
+                margin-bottom: 0.5rem;
+            }
+            
+            .skeleton-text.title {
+                height: 2rem;
+                width: 60%;
+            }
+            
+            .skeleton-text.subtitle {
+                height: 1.2rem;
+                width: 40%;
+            }
+            
+            .skeleton-card {
+                height: 120px;
+                margin-bottom: 1rem;
+            }
+            
+            .skeleton-button {
+                height: 2.5rem;
+                width: 120px;
+                border-radius: 6px;
+            }
+            
+            .dark .skeleton {
+                background: linear-gradient(90deg, #374151 25%, #4b5563 50%, #374151 75%);
+                background-size: 200% 100%;
+            }
+        `;
+        
+        this.init();
+    }
+    
+    init() {
+        // Inject skeleton styles
+        if (!document.getElementById('skeleton-styles')) {
+            const style = document.createElement('style');
+            style.id = 'skeleton-styles';
+            style.textContent = this.skeletonStyles;
+            document.head.appendChild(style);
+        }
+    }
+    
+    // Create skeleton element
+    createSkeleton(type = 'text', customClass = '') {
+        const skeleton = document.createElement('div');
+        skeleton.className = `skeleton skeleton-${type} ${customClass}`;
+        return skeleton;
+    }
+    
+    // Show skeleton for a container
+    showSkeleton(container, config = {}) {
+        const {
+            type = 'text',
+            count = 3,
+            customClass = ''
+        } = config;
+        
+        // Store original content
+        const originalContent = container.innerHTML;
+        container.dataset.originalContent = originalContent;
+        
+        // Clear and add skeletons
+        container.innerHTML = '';
+        for (let i = 0; i < count; i++) {
+            container.appendChild(this.createSkeleton(type, customClass));
+        }
+    }
+    
+    // Hide skeleton and restore content
+    hideSkeleton(container, content = null) {
+        if (content) {
+            container.innerHTML = content;
+        } else if (container.dataset.originalContent) {
+            container.innerHTML = container.dataset.originalContent;
+            delete container.dataset.originalContent;
+        }
+    }
+    
+    // Marketplace specific skeletons
+    showMarketplaceSkeletons() {
+        const statsContainer = document.querySelector('#stats-container');
+        const offersContainer = document.querySelector('#offers-container');
+        
+        if (statsContainer) {
+            this.showSkeleton(statsContainer, {
+                type: 'card',
+                count: 4,
+                customClass: 'stats-skeleton'
+            });
+        }
+        
+        if (offersContainer) {
+            this.showSkeleton(offersContainer, {
+                type: 'card',
+                count: 6,
+                customClass: 'offer-skeleton'
+            });
+        }
+    }
+}
+
+// Initialize skeleton loader
+window.skeletonLoader = new SkeletonLoader();
+
+// Auto-hide skeletons when content is loaded
+document.addEventListener('DOMContentLoaded', () => {
+    // Hide skeletons after a timeout as fallback
+    setTimeout(() => {
+        document.querySelectorAll('.skeleton').forEach(skeleton => {
+            const container = skeleton.parentElement;
+            if (container && container.dataset.originalContent) {
+                window.skeletonLoader.hideSkeleton(container);
+            }
+        });
+    }, 5000);
+});