fix(frontend): sign out via Clerk when in Clerk mode

Sign-out only cleared the local cm_session cookie via DELETE /auth/session.
In Clerk mode the real session is Clerk's, so that left it intact and the
push to /login bounced the user straight back into the app (Clerk's <SignIn>
auto-redirects when a session exists). Detect clerk-js at runtime
(window.Clerk) and sign out through it; local/air-gapped builds have no
window.Clerk and keep the cookie flow (never loading the Clerk SDK).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

frontend/src/components/shell/UserMenu.tsx

@@ -13,6 +13,21 @@ export function UserMenu({ user }: { user: User }) {
 
   async function signOut() {
     setPending(true);
+    // In Clerk mode the real session is Clerk's, not the local cm_session
+    // cookie — so clear it via the Clerk client (clerk-js exposes window.Clerk).
+    // The local /auth/session DELETE only drops the local cookie and would
+    // leave the Clerk session intact, bouncing the user right back into the
+    // app. window.Clerk is absent in local/air-gapped builds, which fall
+    // through to the cookie flow (and never load the Clerk SDK).
+    const clerk = (
+      globalThis as {
+        Clerk?: { signOut: (opts?: { redirectUrl?: string }) => Promise<void> };
+      }
+    ).Clerk;
+    if (clerk) {
+      await clerk.signOut({ redirectUrl: "/login" });
+      return;
+    }
     await fetch("/auth/session", { method: "DELETE" });
     router.push("/login");
     router.refresh();