style: fix unused variable warnings

Prefix unused error variables with underscore to indicate
intentionally unused catch parameters.

src/app/auth/signin/page.tsx

@@ -26,7 +26,7 @@ export default function SignInPage() {
         providerId: "authentik",
         callbackURL: "/",
       })
-    } catch (error) {
+    } catch (_error) {
       toast.error("Failed to sign in. Please try again.")
     } finally {
       setIsLoading(false)