feat(upload): wire A8 local track upload to backend

Implement the A8 upload screen against the existing /upload contract:
- UploadResponse type ({track_id, title, already_exists}) + mutation typed to it
- buildUploadFormData helper (single file under field `file`, per FastAPI)
- UploadPage: drag-and-drop + file picker, client-side queue with
  concurrency cap (3), per-file status badges, retry on error,
  already_exists -> "Already in library", deep-link to A7 metadata editor
- i18n upload.* section (en/ru) incl. "metadata pending" hint

Indeterminate spinner per file; percent progress is a follow-up
(needs an XHR baseQuery — fetchBaseQuery gives no upload progress).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/types.ts

@@ -72,6 +72,12 @@ export interface DownloadJob {
   updatedAt: string;
 }
 
+export interface UploadResponse {
+  track_id: string;
+  title: string;
+  already_exists: boolean;
+}
+
 export interface StorageStats {
   totalBytes: number;
   usedBytes: number;