fix(frontend): Linting errors/warnings opgelost

frontend/src/views/CallbackPage.vue

@@ -1,22 +1,25 @@
 <script setup lang="ts">
     import { useRouter } from "vue-router";
-    import { onMounted } from "vue";
+    import {onMounted, ref, type Ref} from "vue";
     import auth from "../services/auth/auth-service.ts";
 
     const router = useRouter();
 
+    const errorMessage: Ref<string | null> = ref(null);
+
     onMounted(async () => {
         try {
             await auth.handleLoginCallback();
             await router.replace("/user"); // Redirect to theme page
-        } catch (_error) {
-            // FIXME console.error("OIDC callback error:", error);
+        } catch (error) {
+            errorMessage.value = `OIDC callback error: ${error}`;
         }
     });
 </script>
 
 <template>
-    <p>Logging you in...</p>
+    <p v-if="!errorMessage">Logging you in...</p>
+    <p v-else>{{ errorMessage }}</p>
 </template>
 
 <style scoped></style>