fix: auth errors

- Fixes Discord not logging in
- Also shows the login page but with an error message for future errors

frontend/src/pages/account-error.tsx

@@ -0,0 +1,37 @@
+const ERRORS: Record<string, { title: string; messages: string[] }> = {
+	OAuthAccountNotLinked: {
+		title: "Account already exists",
+		messages: [
+			"An account with this email already exists, but it was created using a different login method.",
+			"Please sign in with the provider you originally used (Google, GitHub, or Discord).",
+		],
+	},
+	Configuration: {
+		title: "Server error",
+		messages: ["There is a problem with the server configuration. Please e-mail hello@trafficlunar.net or message @trafficlunar on Discord"],
+	},
+};
+
+const FALLBACK = {
+	title: "Sign in failed",
+	messages: ["Something went wrong while signing in. Please try again."],
+};
+
+export default function AccountErrorPage() {
+	const code = new URLSearchParams(window.location.search).get("error") ?? "";
+	const { title, messages } = ERRORS[code] ?? FALLBACK;
+
+	return (
+		<div className="grow flex items-center justify-center">
+			<div className="bg-amber-50 border-2 border-amber-500 rounded-2xl shadow-lg p-8 max-w-lg w-full text-center flex flex-col gap-2">
+				<h2 className="text-3xl font-black">{title}</h2>
+				{messages.map((message) => (
+					<p key={message}>{message}</p>
+				))}
+				<a href="/" className="mt-4 mx-auto px-4 py-2 rounded-lg bg-amber-500 text-white font-bold hover:bg-amber-600 transition-colors">
+					Back to home
+				</a>
+			</div>
+		</div>
+	);
+}