Add brain and chat workspace views

Expand the frontend with brain, graph, and chat workspace updates so the
new backend orchestration and memory features have matching screens.
These changes also wire the new APIs into routing and add focused view
and routing tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/app/navigation/nav.ts

@@ -24,7 +24,7 @@ export const navItems: NavItem[] = [
   { name: '智能链路', path: '/agents', icon: Bot },
   { name: '技能中心', path: '/skills', icon: Star },
   { name: '资料中枢', path: '/knowledge', icon: BookOpen },
-  { name: '知识大脑', path: '/graph', icon: Network },
+  { name: '知识大脑', path: '/brain', icon: Network },
   { name: '任务矩阵', path: '/kanban', icon: LayoutGrid },
   { name: '任务调度', path: '/todo', icon: CheckSquare },
   { name: '信息交易所', path: '/forum', icon: MessageSquare },