refactor(App): enhance vertical sidebar layout and styling for improved usability and consistency

frontend/src/App.css

@@ -54,6 +54,28 @@
 .navbar-vertical > .container-fluid {
   padding-left: 1rem;
   padding-right: 1rem;
+  display: flex;
+  flex-direction: column;
+  min-height: 100%;
+}
+.navbar-vertical .collapse.navbar-collapse {
+  display: flex;
+  flex-direction: column;
+  flex: 1;
+  min-height: 0;
+}
+.navbar-vertical .collapse .navbar-nav.pt-lg-3 {
+  flex: 1 1 auto;
+  min-height: 0;
+  overflow-y: auto;
+}
+.navbar-vertical .navbar-sidebar-footer {
+  flex-shrink: 0;
+  border-top-color: rgba(255, 255, 255, 0.1) !important;
+}
+.navbar-vertical .navbar-sidebar-footer .nav-link {
+  padding-left: 0.75rem;
+  padding-right: 0.75rem;
 }
 /* Стрелка только у пунктов с подменю (dropdown-toggle), не у обычных ссылок и не у бренда */
 .navbar-vertical .nav-link:not(.dropdown-toggle)::after {