fix(auth): move Google callback page to correct route path

The callback page was placed under app/(auth)/callback/ — a Next.js
route group — which mapped to /callback instead of /auth/callback.
Move it to app/auth/callback/ so the URL matches the Google OAuth
redirect URI.

apps/web/app/auth/callback/page.tsx

@@ -0,0 +1,90 @@
+"use client";
+
+import { Suspense, useEffect, useState } from "react";
+import { useSearchParams, useRouter } from "next/navigation";
+import { useAuthStore } from "@/features/auth";
+import { useWorkspaceStore } from "@/features/workspace";
+import { api } from "@/shared/api";
+import {
+  Card,
+  CardHeader,
+  CardTitle,
+  CardDescription,
+  CardContent,
+} from "@/components/ui/card";
+import { Loader2 } from "lucide-react";
+
+function CallbackContent() {
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const loginWithGoogle = useAuthStore((s) => s.loginWithGoogle);
+  const hydrateWorkspace = useWorkspaceStore((s) => s.hydrateWorkspace);
+  const [error, setError] = useState("");
+
+  useEffect(() => {
+    const code = searchParams.get("code");
+    if (!code) {
+      setError("Missing authorization code");
+      return;
+    }
+
+    const errorParam = searchParams.get("error");
+    if (errorParam) {
+      setError(errorParam === "access_denied" ? "Access denied" : errorParam);
+      return;
+    }
+
+    const redirectUri = `${window.location.origin}/auth/callback`;
+
+    loginWithGoogle(code, redirectUri)
+      .then(async () => {
+        const wsList = await api.listWorkspaces();
+        const lastWsId = localStorage.getItem("multica_workspace_id");
+        await hydrateWorkspace(wsList, lastWsId);
+        router.push("/issues");
+      })
+      .catch((err) => {
+        setError(err instanceof Error ? err.message : "Login failed");
+      });
+  }, [searchParams, loginWithGoogle, hydrateWorkspace, router]);
+
+  if (error) {
+    return (
+      <div className="flex min-h-screen items-center justify-center">
+        <Card className="w-full max-w-sm">
+          <CardHeader className="text-center">
+            <CardTitle className="text-2xl">Login Failed</CardTitle>
+            <CardDescription>{error}</CardDescription>
+          </CardHeader>
+          <CardContent className="flex justify-center">
+            <a href="/login" className="text-primary underline-offset-4 hover:underline">
+              Back to login
+            </a>
+          </CardContent>
+        </Card>
+      </div>
+    );
+  }
+
+  return (
+    <div className="flex min-h-screen items-center justify-center">
+      <Card className="w-full max-w-sm">
+        <CardHeader className="text-center">
+          <CardTitle className="text-2xl">Signing in...</CardTitle>
+          <CardDescription>Please wait while we complete your login</CardDescription>
+        </CardHeader>
+        <CardContent className="flex justify-center">
+          <Loader2 className="h-6 w-6 animate-spin text-muted-foreground" />
+        </CardContent>
+      </Card>
+    </div>
+  );
+}
+
+export default function CallbackPage() {
+  return (
+    <Suspense fallback={null}>
+      <CallbackContent />
+    </Suspense>
+  );
+}