feat: refactor authentication panels and add AdminOnlyNotice component

frontend/src/components/auth/AdminOnlyNotice.tsx

@@ -0,0 +1,11 @@
+type AdminOnlyNoticeProps = {
+  message: string;
+};
+
+export function AdminOnlyNotice({ message }: AdminOnlyNoticeProps) {
+  return (
+    <div className="rounded-xl border border-slate-200 bg-white px-6 py-5 text-sm text-slate-600 shadow-sm">
+      {message}
+    </div>
+  );
+}

frontend/src/components/auth/SignedOutPanel.tsx

@@ -0,0 +1,26 @@
+import { SignInButton } from "@/auth/clerk";
+
+import { Button } from "@/components/ui/button";
+
+type SignedOutPanelProps = {
+  message: string;
+  forceRedirectUrl: string;
+  buttonLabel?: string;
+};
+
+export function SignedOutPanel({
+  message,
+  forceRedirectUrl,
+  buttonLabel = "Sign in",
+}: SignedOutPanelProps) {
+  return (
+    <div className="col-span-2 flex min-h-[calc(100vh-64px)] items-center justify-center bg-slate-50 p-10 text-center">
+      <div className="rounded-xl border border-slate-200 bg-white px-8 py-6 shadow-sm">
+        <p className="text-sm text-slate-600">{message}</p>
+        <SignInButton mode="modal" forceRedirectUrl={forceRedirectUrl}>
+          <Button className="mt-4">{buttonLabel}</Button>
+        </SignInButton>
+      </div>
+    </div>
+  );
+}