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/endpoints/upload.ts

@@ -1,9 +1,19 @@
 import { api } from '../index';
-import type { Track } from '../types';
+import type { UploadResponse } from '../types';
+
+/**
+ * Build the multipart body for `/upload`. The backend expects exactly one file
+ * per request under the field name `file` (anything else → FastAPI 422).
+ */
+export function buildUploadFormData(file: File): FormData {
+  const fd = new FormData();
+  fd.append('file', file);
+  return fd;
+}
 
 export const uploadApi = api.injectEndpoints({
   endpoints: (build) => ({
-    uploadTrack: build.mutation<Track, FormData>({
+    uploadTrack: build.mutation<UploadResponse, FormData>({
       query: (formData) => ({
         url: '/upload',
         method: 'POST',