[refactor] Rename queueIndex variables to reflect job.priority usage (#9258)

## Summary
Rename `lastHistoryQueueIndex` → `lastJobHistoryPriority` and
`currentQueueIndex` → `currentJobPriority` to reflect that these
variables now read `job.priority` directly.

## Changes
- **queueStore.ts**: `lastHistoryQueueIndex` → `lastJobHistoryPriority`
- **JobDetailsPopover.vue**: `currentQueueIndex` → `currentJobPriority`
- **queueStore.test.ts**: Updated references and test descriptions

Fixes #9246

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9258-refactor-Rename-queueIndex-variables-to-reflect-job-priority-usage-3136d73d36508126989dd464f7dad9a1)
by [Unito](https://www.unito.io)

src/components/queue/job/JobDetailsPopover.vue

@@ -166,13 +166,13 @@ const queuedAtValue = computed(() =>
     : ''
 )
 
-const currentQueueIndex = computed<number | null>(() => {
+const currentJobPriority = computed<number | null>(() => {
   const task = taskForJob.value
   return task ? Number(task.job.priority) : null
 })
 
 const jobsAhead = computed<number | null>(() => {
-  const idx = currentQueueIndex.value
+  const idx = currentJobPriority.value
   if (idx == null) return null
   const ahead = queueStore.pendingTasks.filter(
     (t: TaskItemImpl) => Number(t.job.priority) < idx