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/sw.js

@@ -0,0 +1,69 @@
+const CACHE_NAME = 'aitbc-v1';
+const urlsToCache = [
+  '/',
+  '/index.html',
+  '/assets/css/main.css',
+  '/assets/css/font-awesome.min.css',
+  '/assets/js/main.js',
+  '/favicon.ico',
+  '/explorer/',
+  '/marketplace/',
+  '/docs/'
+];
+
+// Install event - cache assets
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(CACHE_NAME)
+      .then(cache => {
+        console.log('Caching assets...');
+        return cache.addAll(urlsToCache);
+      })
+      .catch(err => console.log('Cache failed:', err))
+  );
+  self.skipWaiting();
+});
+
+// Fetch event - serve from cache or network
+self.addEventListener('fetch', event => {
+  event.respondWith(
+    caches.match(event.request)
+      .then(response => {
+        // Return cached version or fetch from network
+        if (response) {
+          return response;
+        }
+        return fetch(event.request)
+          .then(networkResponse => {
+            // Cache successful network responses
+            if (networkResponse && networkResponse.status === 200) {
+              const clonedResponse = networkResponse.clone();
+              caches.open(CACHE_NAME).then(cache => {
+                cache.put(event.request, clonedResponse);
+              });
+            }
+            return networkResponse;
+          })
+          .catch(() => {
+            // Return offline fallback for HTML requests
+            if (event.request.mode === 'navigate') {
+              return caches.match('/index.html');
+            }
+          });
+      })
+  );
+});
+
+// Activate event - clean up old caches
+self.addEventListener('activate', event => {
+  event.waitUntil(
+    caches.keys().then(cacheNames => {
+      return Promise.all(
+        cacheNames
+          .filter(name => name !== CACHE_NAME)
+          .map(name => caches.delete(name))
+      );
+    })
+  );
+  self.clients.claim();
+});