web: guard getExplorePageData against null QueueConfig

getQueueConfig returns null when the queue name is unknown.
Dereferencing it immediately caused an NPE that surfaced as an
HTTP 500 in the host app.  Return a code=1 error response instead.

Assisted-By: Claude Code

Author: sonus21

rqueue-web/src/main/java/com/github/sonus21/rqueue/web/service/RqueueQDetailServiceImpl.java

@@ -397,6 +397,10 @@ public DataViewResponse getExplorePageData(
       int itemPerPage) {
     QueueConfig queueConfig = rqueueSystemManagerService.getQueueConfig(src);
     DataViewResponse response = new DataViewResponse();
+    if (queueConfig == null) {
+      response.set(1, "Queue '" + src + "' does not exist");
+      return response;
+    }
     boolean deadLetterQueue = queueConfig.isDeadLetterQueue(name);
     boolean scheduledQueue = queueConfig.getScheduledQueueName().equals(name);
     boolean completionQueue = name.equals(queueConfig.getCompletedQueueName());