implement auth error page and moved auth-related pages to /auth path

src/app/auth/signin/page.tsx

@@ -0,0 +1,45 @@
+import { signIn, auth } from "@/auth"
+import { Button } from "@/components/ui/button"
+import { Card, CardContent, CardDescription, CardHeader, CardTitle } from "@/components/ui/card"
+import { redirect } from "next/navigation"
+import Link from "next/link"
+
+export default async function SignInPage() {
+  const session = await auth()
+
+  // If already signed in, redirect to home
+  if (session?.user) {
+    redirect("/")
+  }
+
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-background p-4">
+      <Card className="w-full max-w-md">
+        <CardHeader className="text-center">
+          <CardTitle className="text-2xl font-bold">Welcome</CardTitle>
+          <CardDescription>
+            Sign in to access AI-powered calendar features
+          </CardDescription>
+        </CardHeader>
+        <CardContent className="space-y-4">
+          <form
+            action={async () => {
+              "use server"
+              await signIn("authentik", { redirectTo: "/" })
+            }}
+          >
+            <Button type="submit" className="w-full" size="lg">
+              Continue with Authentik
+            </Button>
+          </form>
+
+          <div className="text-center">
+            <Link href="/" className="text-sm text-muted-foreground hover:underline">
+              Continue without signing in
+            </Link>
+          </div>
+        </CardContent>
+      </Card>
+    </div>
+  )
+}