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

admin/theme/default/views/pages/dashboard.twig

@@ -17,7 +17,7 @@
             <div class="card-body d-flex justify-content-between align-items-center">
                 <div>
                     <h6 class="text-muted mb-1">Weergaven (30 dagen)</h6>
-                    <h3 class="mb-0">{{ (analytics_summary.totals.views ?? 0)|number_format(0, ',', '.') }}</h3>
+                    <h3 class="mb-0">{{ (analytics_summary.totals.views|default(0))|number_format(0, ',', '.') }}</h3>
                 </div>
                 <i class="bi bi-eye stat-icon text-primary"></i>
             </div>
@@ -28,7 +28,7 @@
             <div class="card-body d-flex justify-content-between align-items-center">
                 <div>
                     <h6 class="text-muted mb-1">Unieke bezoekers (30 dagen)</h6>
-                    <h3 class="mb-0">{{ (analytics_summary.totals.uniques ?? 0)|number_format(0, ',', '.') }}</h3>
+                    <h3 class="mb-0">{{ (analytics_summary.totals.uniques|default(0))|number_format(0, ',', '.') }}</h3>
                 </div>
                 <i class="bi bi-people stat-icon text-success"></i>
             </div>