show information to user to check if info is valid

templates/confirm.php

@@ -25,6 +25,11 @@ $h = static function (string $value): string {
                 Nextcloud Calendar.
             </p>
 
+            <p class="atocal-notice">
+                This request came from an external link.
+                Only continue if you trust its source.
+            </p>
+
             <?php if ($_['error'] !== ''): ?>
                 <div class="atocal-error">
                     <?= $h($_['error']) ?>

templates/style.css

@@ -22,6 +22,14 @@
     margin-bottom: 28px;
 }
 
+.atocal-notice {
+    background: var(--color-warning);
+    color: var(--color-warning-text);
+    border-radius: 6px;
+    padding: 12px 16px;
+    margin-bottom: 28px;
+}
+
 .atocal-error {
     background: var(--color-error);
     color: var(--color-error-text);