feat(web): alphabet rail chases pages on click for unloaded letters

Operator: prior rail disabled empty buckets, so clicking a letter
like 'Z' did nothing if it wasn't loaded yet. AlphabeticalGrid now
accepts a paginated source and walks pages on click until the
target letter surfaces.

- New props: hasMore, onLoadMore. When hasMore=true, every empty
  bucket stays enabled (clicking will chase pages); when hasMore=
  false, only populated buckets are clickable.
- jumpTo(bucket): if populated, scrollIntoView. Otherwise loop
  onLoadMore + tick() until the bucket appears or no more pages.
  Loader2 spinner replaces the letter on the pending button;
  cursor:wait + aria-busy. Other rail buttons disable while one is
  pending so clicks don't stack.
- Library Artists + Albums pass hasMore + onLoadMore through to the
  grid. Existing InfiniteScrollSentinel still handles scroll-driven
  loading.

Test: new case asserts rail enables empty buckets when hasMore=true
(the click would trigger onLoadMore).

web/src/lib/components/AlphabeticalGrid.svelte

@@ -1,14 +1,25 @@
 <script lang="ts" generics="T">
   import type { Snippet } from 'svelte';
+  import { tick } from 'svelte';
+  import { Loader2 } from 'lucide-svelte';
 
   let {
     items,
     getKey,
-    item
+    item,
+    hasMore = false,
+    onLoadMore
   }: {
     items: T[];
     getKey: (it: T) => string;
     item: Snippet<[T]>;
+    // When true, clicking a rail letter that's not in the currently
+    // loaded items will trigger onLoadMore() repeatedly until that
+    // letter surfaces (or no more pages exist). Lets a paginated
+    // source like createArtistsQuery still feel like a full jump
+    // rail without eager-loading the whole dataset.
+    hasMore?: boolean;
+    onLoadMore?: () => Promise<unknown> | unknown;
   } = $props();
 
   // Bucket the first character into one of three classes so the
@@ -56,10 +67,42 @@
   const ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('');
   const railEntries: string[] = ['#', ...ALPHABET, '&'];
 
-  function jumpTo(bucket: string) {
+  // Bucket the caller is currently chasing via onLoadMore. Used to
+  // show a spinner on the rail button while pages stream in.
+  let pendingBucket = $state<string | null>(null);
+
+  function scrollNow(bucket: string) {
     const el = document.getElementById(`alpha-${bucket}`);
     el?.scrollIntoView({ behavior: 'smooth', block: 'start' });
   }
+
+  async function jumpTo(bucket: string) {
+    // Already loaded — jump in the same frame.
+    if (populated.has(bucket)) {
+      scrollNow(bucket);
+      return;
+    }
+    // No paginated source wired in — nothing to load.
+    if (!hasMore || !onLoadMore) return;
+    // Don't stack loaders.
+    if (pendingBucket !== null) return;
+
+    pendingBucket = bucket;
+    try {
+      // Walk pages until the bucket appears or we exhaust the
+      // dataset. `populated` is $derived from `items`; `tick()`
+      // forces Svelte to flush the prop update before we re-read
+      // populated for the loop condition.
+      // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+      while (hasMore && !populated.has(bucket)) {
+        await onLoadMore();
+        await tick();
+      }
+      if (populated.has(bucket)) scrollNow(bucket);
+    } finally {
+      pendingBucket = null;
+    }
+  }
 </script>
 
 <div class="alpha-layout">
@@ -80,17 +123,28 @@
          vertically centered via position:sticky + top:50%. -->
     <nav class="alpha-rail" aria-label="Jump to letter">
       {#each railEntries as entry}
-        {@const enabled = populated.has(entry)}
+        {@const isLoaded = populated.has(entry)}
+        {@const couldLoad = !isLoaded && hasMore}
+        {@const enabled = isLoaded || couldLoad}
+        {@const isPending = pendingBucket === entry}
         <button
           type="button"
           class="rail-btn"
           class:disabled={!enabled}
-          disabled={!enabled}
+          class:pending={isPending}
+          disabled={!enabled || pendingBucket !== null}
           onclick={() => enabled && jumpTo(entry)}
           aria-label={enabled
             ? `Jump to ${entry === '#' ? 'numbers' : entry === '&' ? 'symbols' : entry}`
             : `${entry === '#' ? 'Numbers' : entry === '&' ? 'Symbols' : entry} — no entries`}
-        >{entry}</button>
+          aria-busy={isPending}
+        >
+          {#if isPending}
+            <Loader2 size={11} strokeWidth={2} class="spin" aria-hidden="true" />
+          {:else}
+            {entry}
+          {/if}
+        </button>
       {/each}
     </nav>
   {/if}
@@ -146,4 +200,14 @@
     color: color-mix(in srgb, var(--fs-ash) 50%, transparent);
     cursor: default;
   }
+  .rail-btn.pending {
+    color: var(--fs-accent);
+    cursor: wait;
+  }
+  :global(.rail-btn .spin) {
+    animation: spin 1s linear infinite;
+  }
+  @keyframes spin {
+    to { transform: rotate(360deg); }
+  }
 </style>

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

@@ -23,27 +23,43 @@ const itemSnippet = createRawSnippet<[Item]>((getIt) => ({
 type AnyProps = any;
 
 describe('AlphabeticalGrid', () => {
-  test('rail renders the full #/A-Z/& set; empty buckets are disabled', () => {
+  test('rail renders the full #/A-Z/& set; empty buckets are disabled when no more data could load', () => {
     render(AlphabeticalGrid, {
       props: {
         items,
         getKey: (it: Item) => it.key,
         item: itemSnippet
+        // hasMore default = false → empty buckets stay disabled.
       } as AnyProps
     });
     // Populated buckets are enabled with 'Jump to <letter>' label.
     expect(screen.getByRole('button', { name: 'Jump to A' })).not.toBeDisabled();
     expect(screen.getByRole('button', { name: 'Jump to B' })).not.toBeDisabled();
     expect(screen.getByRole('button', { name: 'Jump to D' })).not.toBeDisabled();
-    // Empty buckets still render but are disabled and labelled
-    // 'X — no entries' so screen readers announce the empty state.
+    // Empty buckets disabled when nothing more could load. aria-label
+    // changes too so screen readers announce the empty state.
     expect(screen.getByRole('button', { name: 'C — no entries' })).toBeDisabled();
     expect(screen.getByRole('button', { name: 'Z — no entries' })).toBeDisabled();
-    // # (numbers) and & (symbols) always present too.
     expect(screen.getByRole('button', { name: 'Numbers — no entries' })).toBeDisabled();
     expect(screen.getByRole('button', { name: 'Symbols — no entries' })).toBeDisabled();
   });
 
+  test('with hasMore=true, empty buckets are enabled (clicking would trigger onLoadMore)', () => {
+    render(AlphabeticalGrid, {
+      props: {
+        items,
+        getKey: (it: Item) => it.key,
+        item: itemSnippet,
+        hasMore: true,
+        onLoadMore: () => Promise.resolve()
+      } as AnyProps
+    });
+    // Even with no C/Z items loaded, the buttons remain enabled so a
+    // click can chase pages until the letter surfaces.
+    expect(screen.getByRole('button', { name: 'Jump to C' })).not.toBeDisabled();
+    expect(screen.getByRole('button', { name: 'Jump to Z' })).not.toBeDisabled();
+  });
+
   test('renders items in given order followed by the jump rail', () => {
     const { container } = render(AlphabeticalGrid, {
       props: {

web/src/routes/library/albums/+page.svelte

@@ -73,6 +73,8 @@
     <AlphabeticalGrid
       items={filtered}
       getKey={(a: AlbumRef) => a.sort_title || a.title}
+      hasMore={!!query.hasNextPage}
+      onLoadMore={() => query.fetchNextPage()}
     >
       {#snippet item(album: AlbumRef)}
         <AlbumCard {album} />

web/src/routes/library/artists/+page.svelte

@@ -71,6 +71,8 @@
     <AlphabeticalGrid
       items={filtered}
       getKey={(a: ArtistRef) => a.sort_name || a.name}
+      hasMore={!!query.hasNextPage}
+      onLoadMore={() => query.fetchNextPage()}
     >
       {#snippet item(a: ArtistRef)}
         <ArtistCard artist={a} />