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/i18n/locales/en.ts

@@ -179,6 +179,28 @@ const en = {
     description: "This screen doesn't exist yet.",
     backToLibrary: 'Back to library',
   },
+  upload: {
+    title: 'Upload files',
+    dropzone: {
+      title: 'Drag & drop audio files here',
+      hint: 'or click to choose files — one or many at a time',
+      button: 'Choose files',
+    },
+    queueTitle: 'Uploads ({{completed}}/{{total}})',
+    clearCompleted: 'Clear completed',
+    retry: 'Retry',
+    editMetadata: 'Edit metadata',
+    metadataPending:
+      'Uploaded tracks land as “Unknown Artist” with metadata pending — enrich them afterwards.',
+    unknownArtist: 'Unknown Artist · metadata pending',
+    status: {
+      queued: 'Queued',
+      uploading: 'Uploading',
+      done: 'Uploaded',
+      duplicate: 'Already in library',
+      error: 'Failed',
+    },
+  },
 } as const;
 
 export default en;