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/styles/global.scss

@@ -580,6 +580,8 @@ a {
 }
 
 // Shared page header
+// 页面顶栏：发丝线分隔 + 极淡的青色底，标题用等宽字体带字距。
+// 高度保持 64px（ui-baseline 契约，测试有断言）。
 .page-header {
   display: flex;
   align-items: center;
@@ -587,15 +589,20 @@ a {
   gap: 12px;
   min-height: 64px;
   padding: 14px 20px;
-  border-bottom: 1px solid $border-color;
+  border-bottom: 1px solid var(--border-light);
   flex-shrink: 0;
+  background: linear-gradient(180deg,
+    rgba(var(--accent-primary-rgb), 0.035),
+    transparent 70%);
 }
 
 .header-title {
   margin: 0;
-  font-size: 16px;
+  font-family: $font-code;
+  font-size: 15px;
   font-weight: 600;
   line-height: 1.4;
+  letter-spacing: 0.04em;
   color: $text-primary;
 }