Fix dropdown menu styling: no rounded corners, no gaps, consistent hover

- border-radius: 0 on all dropdown-menu and dropdown-item
- padding: 0 and margin: 0 on dropdown-menu
- margin-left: 0 on submenu (no gap between parent and child)
- margin-top: -1px on submenu (seamless border overlap)
- CSS hover opens submenu on desktop, click on mobile
- white-space: nowrap on dropdown items
- Fix statistics getFullStats -> getStats
- Fix Twig ?? operator -> default filter on dashboard/statistics
- Asset server (public/asset.php) for production static file serving
- .htaccess rewrite rules for themes/admin/plugins assets

public/admin.php

@@ -916,7 +916,7 @@ function handleSecurity($auth, $config, $twig, $user, $csrf): void
 function handleStatistics($auth, $config, $twig, $user, $csrf, $siteConfig): void
 {
     $analytics = new Analytics($siteConfig['analytics'] ?? []);
-    $stats = $analytics->getFullStats();
+    $stats = $analytics->getStats();
     $route = 'statistics';
 
     echo $twig->render('pages/statistics.twig', [