Fix Docker container prefix textarea not accepting newlines

Added explicit onKeyDown handler to stop event propagation when Enter
is pressed in the ignored container prefixes textarea. This ensures
newlines can be properly entered to separate multiple prefixes.

Related to #625

frontend-modern/src/components/Alerts/ThresholdsTable.tsx

@@ -2769,6 +2769,13 @@ const cancelEdit = () => {
     <textarea
       value={dockerIgnoredInput()}
       onInput={(event) => handleDockerIgnoredChange(event.currentTarget.value)}
+      onKeyDown={(event) => {
+        // Ensure Enter key works in textarea for creating new lines
+        if (event.key === 'Enter') {
+          // Don't prevent default - allow the newline to be inserted
+          event.stopPropagation();
+        }
+      }}
       placeholder="runner-"
       rows={4}
       class="mt-4 w-full rounded-md border border-gray-300 bg-white p-3 text-sm text-gray-900 shadow-sm focus:border-sky-500 focus:outline-none focus:ring-2 focus:ring-sky-200 dark:border-gray-700 dark:bg-gray-800 dark:text-gray-100 dark:focus:border-sky-400 dark:focus:ring-sky-600/40"