carry the neon language into the sidebar and page header

The nav still used the ink palette: flat fills, oversized rows, and a
tinted background with no marker for the active route. Adopt the CYBER
treatment from the prototype — a 2px glowing cyan bar on the left of the
active item, hairline borders instead of full-contrast ones, monospace
group labels with wide tracking, and a faint cyan wash at the top of the
rail so it does not read as a dead dark slab.

PageSidebarNav gets the same active treatment as AppSidebar so the
conversation routes do not feel like a different product.

Header height stays at 64px — style-system.test.ts asserts it as a
layout contract.

packages/client/src/components/hermes/chat/ChatPanel.vue

@@ -2011,7 +2011,7 @@ async function handleSessionModelCustomSubmit() {
 .page-sidebar-top {
   flex-shrink: 0;
   padding: 12px;
-  border-bottom: 1px solid $border-color;
+  border-bottom: 1px solid var(--border-light);
 }
 
 .page-sidebar-tabs {