refactor(download): 重构下载系统，支持暂停/恢复/取消，修复歌词加载

- 新建 DownloadManager 类（主进程），每个任务独立 AbortController 控制
- 新建 Pinia useDownloadStore 作为渲染进程单一数据源
- 支持暂停/恢复/取消下载，支持断点续传（Range header）
- 批量下载全部完成后发送汇总系统通知，单首不重复通知
- 并发数可配置（1-5），队列持久化（重启后恢复）
- 修复下载列表不全、封面加载失败、通知重复等 bug
- 修复本地/下载歌曲歌词加载：优先从 ID3/FLAC 元数据提取，API 作为 fallback
- 删除 useDownloadStatus.ts，统一状态管理
- DownloadDrawer/DownloadPage 全面重写，移除 @apply 违规
- 新增 5 语言 i18n 键值（暂停/恢复/取消/排队中等）

src/preload/index.d.ts

@@ -44,6 +44,25 @@ interface API {
   parseLocalMusicMetadata: (
     filePaths: string[]
   ) => Promise<import('../renderer/types/localMusic').LocalMusicMeta[]>;
+  // Download manager
+  downloadAdd: (task: any) => Promise<string>;
+  downloadAddBatch: (tasks: any) => Promise<{ batchId: string; taskIds: string[] }>;
+  downloadPause: (taskId: string) => Promise<void>;
+  downloadResume: (taskId: string) => Promise<void>;
+  downloadCancel: (taskId: string) => Promise<void>;
+  downloadCancelAll: () => Promise<void>;
+  downloadGetQueue: () => Promise<any[]>;
+  downloadSetConcurrency: (n: number) => void;
+  downloadGetCompleted: () => Promise<any[]>;
+  downloadDeleteCompleted: (filePath: string) => Promise<boolean>;
+  downloadClearCompleted: () => Promise<boolean>;
+  getEmbeddedLyrics: (filePath: string) => Promise<string | null>;
+  downloadProvideUrl: (taskId: string, url: string) => Promise<void>;
+  onDownloadProgress: (cb: (data: any) => void) => void;
+  onDownloadStateChange: (cb: (data: any) => void) => void;
+  onDownloadBatchComplete: (cb: (data: any) => void) => void;
+  onDownloadRequestUrl: (cb: (data: any) => void) => void;
+  removeDownloadListeners: () => void;
 }
 
 // 自定义IPC渲染进程通信接口