fix(web): songs-like play overlay + scrubber smoothing

PlaylistCard: per-artist system variants (songs_like_artist) all share
one variant tag but exist as one playlist per seed artist; routing
their play handler through systemShuffle(variant) hit the wrong
playlist (or 404). Detect via seed_artist_id != null and fall through
to getPlaylist(playlist.id) for those; still tag the queue with the
variant so source attribution stays correct.

smoothPosition.svelte.ts: new useSmoothPosition() hook mirrors
Android's rememberSmoothPositionMs. player.position only updates
~4 Hz (HTML audio timeupdate), so the seek thumb stepped visibly;
$effect resets on each canonical tick / seek / track-change and a
rAF loop extrapolates at playback rate between ticks.

Wired into both PlayerBar.svelte (mini + expanded seek rows) and
now-playing/+page.svelte. Seek input handler still reads the raw
range value (not smoothed.value) so user drags stay authoritative.

web/src/routes/now-playing/+page.svelte

@@ -14,11 +14,16 @@
   } from '$lib/player/store.svelte';
   import { formatDuration } from '$lib/media/duration';
   import { FALLBACK_COVER, coverUrl } from '$lib/media/covers';
+  import { useSmoothPosition } from '$lib/player/smoothPosition.svelte';
   import LikeButton from '$lib/components/LikeButton.svelte';
   import { pageTitle } from '$lib/branding';
 
   const current = $derived(player.current);
 
+  // Per-frame interpolated playhead so the scrubber thumb glides
+  // smoothly between server ticks. Shares the helper with PlayerBar.
+  const smoothed = useSmoothPosition();
+
   const skipPrevDisabled = $derived(player.index === 0 && player.position < 3);
   const skipNextDisabled = $derived(
     player.index === player.queue.length - 1 && player.repeat === 'off'
@@ -123,13 +128,13 @@
           min="0"
           max={player.duration || 0}
           step="0.1"
-          value={player.position}
+          value={smoothed.value}
           oninput={onSeekInput}
           aria-label="Seek"
           class="w-full accent-accent"
         />
         <div class="mt-1 flex justify-between text-xs text-text-secondary tabular-nums">
-          <span>{formatDuration(player.position)}</span>
+          <span>{formatDuration(smoothed.value)}</span>
           <span>{formatDuration(player.duration)}</span>
         </div>
       </div>