Complete demo-mode auth conversion and fix the frontend build

proxy.ts already declared auth disabled, but layout.tsx still wrapped the
tree in ClerkProvider and topbar.tsx rendered UserButton. @clerk/nextjs v7
aborts `next build` without a real publishable key, so the image could not
be built at all. Remove the remaining Clerk surface, drop the dependency,
and delete the sign-in/sign-up route groups.

Also fix two pnpm failures this exposed: the deps stage never copied
pnpm-workspace.yaml (so the build-script allowlist was invisible), and
pnpm 11 renamed ignoredBuiltDependencies to allowBuilds. Declare both
spellings and pin packageManager so this cannot drift again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/components/layout/topbar.tsx

@@ -4,7 +4,6 @@ import { Bell, Menu, Moon, Sun } from "lucide-react";
 import { SimulatorPanel } from "@/components/simulator/SimulatorPanel";
 import { usePathname } from "next/navigation";
 import Link from "next/link";
-import { UserButton } from "@clerk/nextjs";
 import { useTheme } from "next-themes";
 import { Badge } from "@/components/ui/badge";
 import { Button } from "@/components/ui/button";
@@ -85,15 +84,6 @@ export function Topbar() {
         >
           {theme === "dark" ? <Sun className="w-4 h-4" /> : <Moon className="w-4 h-4" />}
         </Button>
-
-        {/* Clerk user button */}
-        <UserButton
-          appearance={{
-            elements: {
-              avatarBox: "w-7 h-7",
-            },
-          }}
-        />
       </div>
     </header>
   );