feat: add dark mode, navigation, and Web Vitals tracking to marketplace

Backend:
- Simplify DatabaseConfig: remove effective_url property and project root finder
- Update to Pydantic v2 model_config (replace nested Config class)
- Add web_vitals router to main.py and __init__.py
- Fix ExplorerService datetime handling (ensure timezone-naive comparisons)
- Fix status_label extraction to handle both enum and string job states

Frontend (Marketplace):
- Add dark mode toggle with system preference detection

website/assets/js/analytics.js

@@ -0,0 +1,53 @@
+// Lightweight privacy-focused analytics for AITBC
+// No cookies, no tracking, just basic page view metrics
+(function() {
+    'use strict';
+    
+    const script = document.currentScript;
+    const host = script.getAttribute('data-host') || window.location.origin;
+    const website = script.getAttribute('data-website') || 'default';
+    
+    // Send page view on load
+    function sendPageView() {
+        const data = {
+            url: window.location.href,
+            pathname: window.location.pathname,
+            hostname: window.location.hostname,
+            referrer: document.referrer,
+            screenWidth: window.screen.width,
+            screenHeight: window.screen.height,
+            language: navigator.language,
+            website: website,
+            timestamp: new Date().toISOString()
+        };
+        
+        // Use sendBeacon for reliable delivery
+        if (navigator.sendBeacon) {
+            navigator.sendBeacon(`${host}/api/analytics`, JSON.stringify(data));
+        } else {
+            // Fallback to fetch
+            fetch(`${host}/api/analytics`, {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify(data),
+                keepalive: true
+            }).catch(() => {});
+        }
+    }
+    
+    // Send initial page view
+    if (document.readyState === 'complete') {
+        sendPageView();
+    } else {
+        window.addEventListener('load', sendPageView);
+    }
+    
+    // Track route changes for SPA
+    let lastPath = window.location.pathname;
+    setInterval(() => {
+        if (window.location.pathname !== lastPath) {
+            lastPath = window.location.pathname;
+            sendPageView();
+        }
+    }, 1000);
+})();