refactor(ServerSidebar): implement new filter section and sidebar styling for improved user interaction and organization

frontend/src/App.css

@@ -256,6 +256,42 @@
 /* Убрали глобальную перекраску .badge, чтобы работали текстовые цвета Tabler
    (например, badge bg-blue-lt text-blue) */
 
+/* --- Server sidebar (фильтры на /servers) — Tabler card + list-group --- */
+.server-sidebar {
+  width: 280px;
+  min-width: 280px;
+  max-height: calc(100vh - 180px);
+  display: flex;
+  flex-direction: column;
+}
+.server-sidebar .card-header {
+  flex-shrink: 0;
+}
+.server-sidebar .card-body.border-bottom,
+.server-sidebar .card-footer {
+  flex-shrink: 0;
+}
+.server-sidebar-body {
+  flex: 1;
+  min-height: 0;
+  max-height: 50vh;
+}
+.server-sidebar .filter-section .list-group-header {
+  cursor: pointer;
+  font-size: 0.8125rem;
+  font-weight: 600;
+  text-transform: none;
+  color: var(--tblr-body-color);
+}
+.server-sidebar .filter-section .list-group-header:hover {
+  background-color: var(--tblr-bg-surface-tertiary);
+}
+.server-sidebar .list-group-item {
+  font-size: 0.8125rem;
+  padding-top: 0.375rem;
+  padding-bottom: 0.375rem;
+}
+
 /* Responsive navbar */
 @media (max-width: 991.98px) {
   .navbar-collapse {