feat(library): focused music/TV/movie landing screens + self-draining download queue

Library screens:
- Add dedicated music, TV, and movie landing pages (hero banner +
  horizontal carousels) backed by new music/tv/movies stores.
- Route tvshows libraries to /library/tv; surface rediscover ("haven't
  listened to in a while") albums via a new repository method across
  online/offline/hybrid repos plus the repository_get_rediscover_albums
  command.
- Add an A-Z jump bar for long alphabetically-sorted lists, with grid
  index anchors in LibraryGrid/LibraryListView/TrackList.
- Filter the "Podcasts" folder out of music library queries.

Downloads:
- Add a backend queue pump: enqueue_download / enqueue_video_downloads
  persist the resolved stream URL + target dir on each row (migration
  017), and the pump starts up to max_concurrent and drains the rest
  automatically as slots free, instead of the frontend silently dropping
  items past the concurrency limit. Album/series/season buttons now
  enqueue rather than calling start_download directly.

Other fixes:
- Hybrid search now returns instant cache results and pushes the merged
  cache+server union via a request-id-tagged search-event, so superseded
  queries can't clobber fresher results.
- URL-encode SearchTerm / genres / item types in online repo requests.
- Android: pause on audio-becoming-noisy (headphone/BT disconnect).

src/lib/components/library/AlphabetScrollBar.svelte

@@ -0,0 +1,123 @@
+<!-- TRACES: UR-007 | DR-007 -->
+<script lang="ts">
+  /**
+   * A vertical A-Z index strip for long, alphabetically-sorted lists.
+   * Tapping or dragging a letter jumps to the first item that starts with it.
+   *
+   * The parent owns the actual scrolling: it passes `availableLetters`
+   * (which letters have items) and an `onJump(letter)` callback.
+   */
+
+  const ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("");
+  const HASH = "#"; // bucket for names starting with a digit/symbol
+
+  interface Props {
+    /** Uppercase letters (or "#") that currently have at least one item. */
+    availableLetters: Set<string>;
+    /** Called with the chosen letter when the user picks one. */
+    onJump: (letter: string) => void;
+    /**
+     * CSS length reserved at the bottom of the viewport for the bottom nav /
+     * mini-player bars. The strip stretches to fill the space between the top
+     * sticky offset and this gap, so it ends just above those bars.
+     */
+    bottomGap?: string;
+  }
+
+  let { availableLetters, onJump, bottomGap = "5rem" }: Props = $props();
+
+  // The strip stretches to fill the height of its scroll container (the page's
+  // <main>, which sits below the sticky header) from the sticky top offset down
+  // to the bottom gap reserved for the nav / mini-player bars. We measure the
+  // container's visible height live so it stays correct regardless of header
+  // size, platform, or window resizes.
+  let container = $state<HTMLDivElement | null>(null);
+  let viewportHeight = $state(0);
+
+  function measure() {
+    const scroller = container?.closest("main") ?? document.documentElement;
+    viewportHeight = scroller.clientHeight;
+  }
+
+  $effect(() => {
+    measure();
+    const scroller = container?.closest("main");
+    const ro = new ResizeObserver(measure);
+    if (scroller) ro.observe(scroller);
+    window.addEventListener("resize", measure);
+    return () => {
+      ro.disconnect();
+      window.removeEventListener("resize", measure);
+    };
+  });
+
+  const letters = $derived([HASH, ...ALPHABET]);
+  let activeLetter = $state<string | null>(null);
+
+  function jumpTo(letter: string) {
+    if (!availableLetters.has(letter)) return;
+    activeLetter = letter;
+    onJump(letter);
+  }
+
+  // Allow dragging a finger/mouse down the strip to scrub through letters.
+  function letterAt(clientY: number): string | null {
+    const el = document.elementFromPoint(
+      // x is supplied by the caller via the bound container; we read from the
+      // element under the pointer instead to stay layout-agnostic.
+      lastClientX,
+      clientY
+    );
+    const letter = el?.getAttribute?.("data-letter");
+    return letter ?? null;
+  }
+
+  let lastClientX = $state(0);
+  let isScrubbing = $state(false);
+
+  function handlePointerDown(e: PointerEvent) {
+    isScrubbing = true;
+    lastClientX = e.clientX;
+    const letter = letterAt(e.clientY);
+    if (letter) jumpTo(letter);
+  }
+
+  function handlePointerMove(e: PointerEvent) {
+    if (!isScrubbing) return;
+    lastClientX = e.clientX;
+    const letter = letterAt(e.clientY);
+    if (letter && letter !== activeLetter) jumpTo(letter);
+  }
+
+  function handlePointerUp() {
+    isScrubbing = false;
+  }
+</script>
+
+<svelte:window onpointerup={handlePointerUp} onpointercancel={handlePointerUp} />
+
+<div
+  bind:this={container}
+  class="flex flex-col items-center justify-between select-none touch-none py-1"
+  style="height: calc({viewportHeight}px - 1.5rem - {bottomGap})"
+  onpointerdown={handlePointerDown}
+  onpointermove={handlePointerMove}
+  role="navigation"
+  aria-label="Jump to letter"
+>
+  {#each letters as letter (letter)}
+    {@const enabled = availableLetters.has(letter)}
+    <button
+      type="button"
+      data-letter={letter}
+      disabled={!enabled}
+      onclick={() => jumpTo(letter)}
+      class="w-5 leading-tight text-[10px] sm:text-xs font-semibold transition-colors
+        {enabled ? 'text-gray-400 hover:text-[var(--color-jellyfin)]' : 'text-gray-700 cursor-default'}
+        {activeLetter === letter && enabled ? 'text-[var(--color-jellyfin)] scale-125' : ''}"
+      aria-label={`Jump to ${letter}`}
+    >
+      {letter}
+    </button>
+  {/each}
+</div>