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

apps/explorer-web/public/css/layout.css

@@ -95,6 +95,20 @@
   gap: 1rem;
 }
 
+.site-header__back {
+  font-size: 0.85rem;
+  padding: 0.25rem 0.6rem;
+  border-radius: 999px;
+  border: 1px solid rgba(125, 196, 255, 0.3);
+  transition: background 150ms ease, border-color 150ms ease;
+  white-space: nowrap;
+}
+
+.site-header__back:hover {
+  background: rgba(125, 196, 255, 0.15);
+  border-color: rgba(125, 196, 255, 0.5);
+}
+
 .site-header__brand {
   font-weight: 600;
   font-size: 1.15rem;

apps/explorer-web/src/components/siteHeader.ts

@@ -4,6 +4,7 @@ export function siteHeader(title: string): string {
   return `
     <header class="site-header">
       <div class="site-header__inner">
+        <a class="site-header__back" href="/" title="Back to AITBC Home">← Home</a>
         <a class="site-header__brand" href="${basePath}/">AITBC Explorer</a>
         <div class="site-header__controls">
           <div data-role="data-mode-toggle"></div>
@@ -14,6 +15,7 @@ export function siteHeader(title: string): string {
           <a href="${basePath}/transactions">Transactions</a>
           <a href="${basePath}/addresses">Addresses</a>
           <a href="${basePath}/receipts">Receipts</a>
+          <a href="/marketplace/">Marketplace</a>
         </nav>
       </div>
     </header>