refactor: 批量 Deep Scan 并发数改为 2，优化排队显示

修改：
- maxConcurrent 从 3 改为 2
- 开始时显示："Starting: 2 immediate, 3 queuing..."
- 处理时显示："Processing 1/5" 或 "Processing 3/5 (1 queuing)..."
- 更清晰的队列状态显示

src/views/LibraryView.vue

@@ -1887,14 +1887,16 @@ async function batchDeepScan() {
   if (!confirm(`Deep scan ${filesToProcess.length} files?\n\nThis will process all selected files with default settings.`)) return
   
   batchProcessing.value = true
-  actionMsg.value = `Starting deep scan for ${filesToProcess.length} files...`
   
-  const maxConcurrent = 3
+  const maxConcurrent = 2
   const limit = pLimit(maxConcurrent)
+  const immediateCount = Math.min(filesToProcess.length, maxConcurrent)
+  const queueTotal = Math.max(0, filesToProcess.length - maxConcurrent)
+  
+  actionMsg.value = `Starting: ${immediateCount} immediate, ${queueTotal} queuing...`
   
   let successCount = 0
   let failCount = 0
-  let queueCount = 0
   
   const tasks = filesToProcess.map((f, index) => {
     return limit(async () => {
@@ -1905,9 +1907,10 @@ async function batchDeepScan() {
       }
       
       const position = index + 1
-      if (position > maxConcurrent) {
-        queueCount++
-        actionMsg.value = `Processing ${successCount + 1}/${filesToProcess.length} (${queueCount} in queue)...`
+      const queuePosition = position - maxConcurrent
+      
+      if (queuePosition > 0) {
+        actionMsg.value = `Processing ${successCount + 1}/${filesToProcess.length} (${queuePosition} queuing)...`
       } else {
         actionMsg.value = `Processing ${position}/${filesToProcess.length}...`
       }