Enhance UI and functionality for admin dashboard and user management

- Update `_base_context` to include `active_nav` for dynamic navigation highlighting.
- Revamp `base.html` to improve header layout and navigation links for better user experience.
- Redesign `dashboard.html` to include statistics for new and edited articles, enhancing visibility of content changes.
- Improve `users.html` layout for user management, adding detailed user roles and status indicators.
- Update CSS styles for a cohesive design across the application, introducing new color variables and layout adjustments.

app/main.py

@@ -93,10 +93,15 @@ def _bootstrap_admin() -> None:
 
 
 def _base_context(request: Request, user: User, db: Session) -> dict:
+    path = request.url.path
+    active_nav = "dashboard"
+    if path.startswith("/admin/users"):
+        active_nav = "users"
     return {
         "user": user,
         "csrf_token": request.cookies.get(CSRF_COOKIE_NAME) or ensure_csrf_cookie(request),
         "smtp": get_smtp_settings(db),
+        "active_nav": active_nav,
     }