fix: replace Firebase signOut with NextAuth signOut in left rail

Firebase is not configured - auth was undefined causing the error
'Cannot read properties of undefined (reading signOut)'.
Use next-auth/react signOut with callbackUrl to redirect to /auth.

Co-authored-by: Cursor <cursoragent@cursor.com>

components/layout/workspace-left-rail.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import Link from "next/link";
-import { usePathname, useRouter } from "next/navigation";
+import { usePathname } from "next/navigation";
 import { cn } from "@/lib/utils";
 import {
   LayoutGrid,
@@ -13,9 +13,7 @@ import {
   LogOut,
 } from "lucide-react";
 import { Separator } from "@/components/ui/separator";
-import { signOut } from "@/lib/firebase/auth";
-import { toast } from "sonner";
-
+import { signOut } from "next-auth/react";
 interface WorkspaceLeftRailProps {
   activeSection?: string;
   onSectionChange: (section: string) => void;
@@ -56,19 +54,12 @@ const navItems = [
 
 export function WorkspaceLeftRail({ activeSection = 'projects', onSectionChange }: WorkspaceLeftRailProps) {
   const pathname = usePathname();
-  const router = useRouter();
 
   // Extract workspace from pathname (e.g., /marks-account/projects -> marks-account)
   const workspace = pathname?.split('/')[1] || 'marks-account';
 
   const handleSignOut = async () => {
-    try {
-      await signOut();
-      toast.success("Signed out successfully");
-      router.push("/auth");
-    } catch (error: any) {
-      toast.error(error.message);
-    }
+    await signOut({ callbackUrl: "/auth" });
   };
 
   return (