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/pnpm-workspace.yaml

@@ -1,3 +1,19 @@
+# pnpm refuses to install (ERR_PNPM_IGNORED_BUILDS) if a dependency has a
+# postinstall script that is neither approved nor explicitly declined.
+#
+# `allowBuilds` is the pnpm 11 spelling; `ignoredBuiltDependencies` is kept for
+# pnpm 10, which does not understand `allowBuilds`. Both must list the same
+# packages. None of these are needed to run `next build`:
+#   sharp         — image-optimisation binary; unused, the app has no next/image
+#   unrs-resolver — native resolver for eslint-config-next (lint only)
+#   msw           — pulled in by the `shadcn` CLI; its postinstall only writes a
+#                   browser service worker
+allowBuilds:
+  msw: false
+  sharp: false
+  unrs-resolver: false
+
 ignoredBuiltDependencies:
   - sharp
   - unrs-resolver
+  - msw