refactor: simplify project nav to AI Chat (overview) + Design only

- AI Chat nav item now routes to /overview instead of /v_ai_chat
- Removed Plan, Docs, Tech, Journey nav items
- Deleted old v_ai_chat page
- Cleaned up unused imports and route detection logic

Made-with: Cursor

components/layout/app-shell.tsx

@@ -18,21 +18,8 @@ export function AppShell({ children, workspace, projectId, projectName }: AppShe
   
   // Derive active section from pathname
   const derivedSection = useMemo(() => {
-    if (pathname.includes('/v_ai_chat')) {
-      return 'ai-chat';
-    } else if (pathname.includes('/overview')) {
-      return 'home';
-    } else if (pathname.includes('/docs')) {
-      return 'docs';
-    } else if (pathname.includes('/plan') || pathname.includes('/timeline-plan')) {
-      return 'plan';
-    } else if (pathname.includes('/design')) {
-      return 'design';
-    } else if (pathname.includes('/tech')) {
-      return 'tech';
-    } else if (pathname.includes('/journey')) {
-      return 'journey';
-    }
+    if (pathname.includes('/overview')) return 'home';
+    if (pathname.includes('/design')) return 'design';
     return activeSection;
   }, [pathname, activeSection]);