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/package.json

@@ -2,6 +2,7 @@
   "name": "frontend",
   "version": "0.1.0",
   "private": true,
+  "packageManager": "pnpm@11.20.0",
   "scripts": {
     "dev": "next dev",
     "build": "next build",
@@ -9,7 +10,6 @@
     "lint": "eslint"
   },
   "dependencies": {
-    "@clerk/nextjs": "^7.0.1",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^0.577.0",