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>
Senko-san
olly