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;