feat(ui): make sure UI shows that commas are not allowed in labels

frontend/public/locales/en/translation.json

@@ -1023,6 +1023,7 @@
       "inUse": "Cannot delete label because it is currently assigned to documents. Please remove the label from all documents first.",
       "systemDelete": "System labels cannot be deleted. Only user-created labels can be removed.",
       "invalidName": "Label name contains invalid characters. Please use only letters, numbers, and basic punctuation.",
+      "commaNotAllowed": "Label names cannot contain commas.",
       "invalidColor": "Invalid color format. Please use a valid hex color like #0969da.",
       "maxLabelsReached": "Maximum number of labels reached. Please delete some labels before creating new ones."
     }

frontend/src/components/Labels/LabelCreateDialog.tsx

@@ -121,6 +121,13 @@ const LabelCreateDialog: React.FC<LabelCreateDialogProps> = ({
       return;
     }
 
+    // Disallow commas in label names (breaks comma-separated search filters)
+    // Also check for URL-encoded commas (%2c) which could cause issues in query parameters
+    if (formData.name.includes(',') || formData.name.toLowerCase().includes('%2c')) {
+      setNameError(t('labels.errors.commaNotAllowed'));
+      return;
+    }
+
     setLoading(true);
     try {
       await onSubmit({