refactor(nav): reorganize sidebar with dynamic sections at top

- move runs and sessions to top of navigation (most dynamic)
- add visual separator between dynamic and configuration sections
- reorder remaining items: scenarios, snippets, credentials, environments
- implement separator component in nav rendering logic

client/src/App.module.css

@@ -95,6 +95,12 @@
   background: var(--color-secondary-hover);
 }
 
+.navSeparator {
+  height: var(--border-width);
+  background: var(--color-border);
+  margin: var(--space-2) var(--space-2);
+}
+
 .main {
   flex: 1;
   overflow-y: auto;