feat(web): add artist detail page at /artists/:id

Renders the artist name + album count and a responsive AlbumCard grid
from the nested ArtistDetail response. 404 surfaces a distinct
non-retryable 'not found' state; other errors share the retry banner.

web/src/routes/artists/[id]/+page.svelte

@@ -0,0 +1,51 @@
+<script lang="ts">
+  import { page } from '$app/state';
+  import { createArtistQuery } from '$lib/api/queries';
+  import AlbumCard from '$lib/components/AlbumCard.svelte';
+  import LibrarySkeleton from '$lib/components/LibrarySkeleton.svelte';
+  import ApiErrorBanner from '$lib/components/ApiErrorBanner.svelte';
+  import { useDelayed } from '$lib/utils/useDelayed.svelte';
+  import type { ApiError } from '$lib/api/client';
+
+  const id = $derived(page.params.id ?? '');
+  const queryStore = $derived(createArtistQuery(id));
+  const query = $derived($queryStore);
+  const showSkeleton = useDelayed(() => query.isPending);
+
+  const notFound = $derived(
+    query.isError && (query.error as unknown as ApiError | undefined)?.code === 'not_found'
+  );
+</script>
+
+<div class="space-y-4">
+  <a href="/" class="text-sm text-text-secondary hover:text-text-primary">← Library</a>
+
+  {#if notFound}
+    <div role="alert" class="rounded border border-border bg-surface p-4">
+      <p>Artist not found.</p>
+      <a href="/" class="mt-2 inline-block text-sm text-accent">Back to Library</a>
+    </div>
+  {:else if query.isError}
+    <ApiErrorBanner error={query.error} onRetry={query.refetch} />
+  {:else if showSkeleton.value && !query.data}
+    <LibrarySkeleton variant="grid" count={12} />
+  {:else if query.data}
+    {@const detail = query.data}
+    <header>
+      <h1 class="text-2xl font-semibold">{detail.name}</h1>
+      <p class="text-sm text-text-secondary">
+        {detail.album_count} {detail.album_count === 1 ? 'album' : 'albums'}
+      </p>
+    </header>
+
+    {#if detail.albums.length === 0}
+      <p class="text-text-secondary">This artist has no albums in the library.</p>
+    {:else}
+      <div class="grid grid-cols-2 gap-4 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-5">
+        {#each detail.albums as album (album.id)}
+          <AlbumCard {album} />
+        {/each}
+      </div>
+    {/if}
+  {/if}
+</div>