feat(web): add /library/artists wrapping-grid page

Implements Task 18: AlphabeticalGrid + ArtistCard infinite-scroll page
backed by createArtistsQuery('alpha'), with error, empty, and loading states.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/routes/library/artists/page.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, test, vi } from 'vitest';
+import { render, screen } from '@testing-library/svelte';
+import { readable } from 'svelte/store';
+
+vi.mock('$lib/api/queries', () => ({
+  createArtistsQuery: () => readable({
+    data: { pages: [{
+      items: [
+        { id: 'a1', name: 'Alpha', sort_name: 'Alpha', album_count: 1, cover_url: '' },
+        { id: 'b1', name: 'Beta',  sort_name: 'Beta',  album_count: 2, cover_url: '' }
+      ],
+      total: 2, limit: 50, offset: 0
+    }] },
+    isPending: false,
+    isError: false,
+    hasNextPage: false,
+    isFetchingNextPage: false,
+    refetch: () => {},
+    fetchNextPage: () => {}
+  })
+}));
+vi.mock('$lib/api/client', () => ({ api: { get: vi.fn() } }));
+vi.mock('$lib/player/store.svelte', () => ({ playQueue: vi.fn() }));
+
+import Page from './+page.svelte';
+
+describe('/library/artists', () => {
+  test('renders title + count + alphabetical dividers', () => {
+    render(Page);
+    expect(screen.getByRole('heading', { name: /artists/i })).toBeInTheDocument();
+    expect(screen.getByText(/2 artists/)).toBeInTheDocument();
+    expect(screen.getByText('A')).toBeInTheDocument();
+    expect(screen.getByText('B')).toBeInTheDocument();
+    expect(screen.getByText('Alpha')).toBeInTheDocument();
+    expect(screen.getByText('Beta')).toBeInTheDocument();
+  });
+});