feat(web): PlaylistCard component for M7 #352 slice 1

Square card with cover (or "No tracks yet" glyph fallback), name,
track count, and owner attribution when the playlist isn't the
current user's. Click navigates to /playlists/{id}.

web/src/lib/components/PlaylistCard.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+  import { goto } from '$app/navigation';
+  import type { Playlist } from '$lib/api/types';
+  import { user } from '$lib/auth/store.svelte';
+
+  let { playlist }: { playlist: Playlist } = $props();
+
+  const isOwn = $derived(user.value?.id === playlist.user_id);
+</script>
+
+<button
+  type="button"
+  onclick={() => goto(`/playlists/${playlist.id}`)}
+  class="group flex w-full flex-col items-start gap-2 rounded-md p-2 text-left hover:bg-surface-hover"
+>
+  <div class="aspect-square w-full overflow-hidden rounded-md bg-surface-hover">
+    {#if playlist.cover_url}
+      <img
+        src={playlist.cover_url}
+        alt=""
+        class="h-full w-full object-cover transition-transform group-hover:scale-105"
+      />
+    {:else}
+      <div class="flex h-full w-full items-center justify-center text-text-muted">
+        <span class="text-xs">No tracks yet</span>
+      </div>
+    {/if}
+  </div>
+  <div class="w-full min-w-0">
+    <div class="truncate text-sm font-medium text-text-primary">{playlist.name}</div>
+    <div class="truncate text-xs text-text-muted">
+      {playlist.track_count} {playlist.track_count === 1 ? 'track' : 'tracks'}
+      {#if !isOwn}
+        · by {playlist.owner_username}
+      {/if}
+    </div>
+  </div>
+</button>

web/src/lib/components/PlaylistCard.test.ts

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+import { describe, expect, test, vi } from 'vitest';
+import { render, screen } from '@testing-library/svelte';
+import PlaylistCard from './PlaylistCard.svelte';
+import type { Playlist } from '$lib/api/types';
+
+vi.mock('$lib/auth/store.svelte', () => ({
+  user: { value: { id: 'u-self', username: 'me', is_admin: false } }
+}));
+
+vi.mock('$app/navigation', () => ({ goto: vi.fn() }));
+
+const base: Playlist = {
+  id: 'p-1',
+  user_id: 'u-self',
+  owner_username: 'me',
+  name: 'Saturday morning',
+  description: '',
+  is_public: false,
+  cover_url: '',
+  track_count: 12,
+  duration_sec: 0,
+  created_at: '',
+  updated_at: ''
+};
+
+describe('PlaylistCard', () => {
+  test('renders own playlist without owner attribution', () => {
+    render(PlaylistCard, { props: { playlist: base } });
+    expect(screen.getByText('Saturday morning')).toBeInTheDocument();
+    expect(screen.getByText(/12\s+tracks/i)).toBeInTheDocument();
+    expect(screen.queryByText(/by /)).not.toBeInTheDocument();
+  });
+
+  test('renders cover image when cover_url is set', () => {
+    render(PlaylistCard, { props: { playlist: { ...base, cover_url: '/api/playlists/p-1/cover' } } });
+    const img = screen.getByRole('img');
+    expect(img.getAttribute('src')).toBe('/api/playlists/p-1/cover');
+  });
+
+  test('renders glyph fallback when cover_url is empty', () => {
+    render(PlaylistCard, { props: { playlist: base } });
+    expect(screen.getByText(/no tracks yet/i)).toBeInTheDocument();
+  });
+
+  test("attributes other users' playlists to the owner", () => {
+    render(PlaylistCard, { props: { playlist: { ...base, user_id: 'u-bob', owner_username: 'bob' } } });
+    expect(screen.getByText(/by bob/i)).toBeInTheDocument();
+  });
+
+  test('singular "track" when count is 1', () => {
+    render(PlaylistCard, { props: { playlist: { ...base, track_count: 1 } } });
+    expect(screen.getByText(/1\s+track\b/i)).toBeInTheDocument();
+  });
+});