feat: 批量 unregister 功能 + 性能优化

功能：
- LibraryView 添加批量 unregister 按钮（勾选多个已注册文件后显示）
- 添加确认对话框，支持批量取消注册
- 添加中英日三语翻译

性能优化：
- search_agents API timeout 提升到 90s（避免前端超时）
- thumbnail 并发数从 32 降到 10（减少服务器压力）

修复：
- PeopleView 添加 group_uuid 字段映射
- LibraryView 排序优化（scanning/preparing 优先）

src/locales/en.json

@@ -49,6 +49,7 @@
       "registered": "Registered",
       "unregistered": "Unregistered",
       "pending": "Pending",
+      "preparing": "Preparing",
       "processing": "Processing",
       "completed": "Completed",
       "scanning": "Scanning",
@@ -136,7 +137,8 @@
       "rerun_processor": "Re-run {processor} for this file?",
       "run_processor": "Run {processor} on {filename}?",
       "unregister_title": "Unregister",
-      "unregister_message": "Unregister \"{filename}\"?\n\nThis will remove the file and its output data."
+      "unregister_message": "Unregister \"{filename}\"?\n\nThis will remove the file and its output data.",
+      "batch_unregister_message": "Unregister {count} files?\n\nThis will remove the files and their output data."
     },
     "processor": {
       "started": "{processor} started",