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/lib/components/ArtistRow.test.ts

@@ -19,7 +19,7 @@ vi.mock('@tanstack/svelte-query', async (orig) => {
   return { ...actual, useQueryClient: () => ({}) };
 });
 
-const artist: ArtistRef = { id: 'abc', name: 'alice', album_count: 12 };
+const artist: ArtistRef = { id: 'abc', name: 'alice', sort_name: 'alice', album_count: 12, cover_url: '' };
 
 describe('ArtistRow', () => {
   test('renders initial, name, album count inside a link to /artists/:id', () => {