feat(web): extend ArtistRef/AlbumRef + add HomePayload + new query keys

Adds sort_name/cover_url to ArtistRef, sort_title to AlbumRef (matching
backend Task 5 wire shape), HomePayload type, and qk.home/albumsAlpha/
artistTracks query keys. Updates existing test fixtures to satisfy the
new required fields.

Verified clean via 'npm run check' (0 errors, only pre-existing warnings).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/routes/artists/[id]/artist.test.ts

@@ -40,6 +40,7 @@ import { createArtistQuery } from '$lib/api/queries';
 function album(id: string, title: string, year?: number): AlbumRef {
   return {
     id, title,
+    sort_title: title,
     artist_id: 'abc', artist_name: 'Alice',
     year, track_count: 10, duration_sec: 2400,
     cover_url: `/api/albums/${id}/cover`
@@ -54,7 +55,7 @@ afterEach(() => {
 describe('artist detail page', () => {
   test('renders artist name, subtitle, and one AlbumCard per album', () => {
     const detail: ArtistDetail = {
-      id: 'abc', name: 'Alice', album_count: 2,
+      id: 'abc', name: 'Alice', sort_name: 'Alice', album_count: 2, cover_url: '',
       albums: [album('a1', 'First', 2020), album('a2', 'Second')]
     };
     (createArtistQuery as ReturnType<typeof vi.fn>).mockReturnValue(mockQuery({ data: detail }));
@@ -67,7 +68,7 @@ describe('artist detail page', () => {
 
   test('back link points to Library', () => {
     (createArtistQuery as ReturnType<typeof vi.fn>).mockReturnValue(mockQuery({
-      data: { id: 'abc', name: 'Alice', album_count: 0, albums: [] }
+      data: { id: 'abc', name: 'Alice', sort_name: 'Alice', album_count: 0, cover_url: '', albums: [] }
     }));
     render(ArtistPage);
     const back = screen.getByRole('link', { name: /library/i });