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/lib/components/PlayerBar.svelte

@@ -14,11 +14,17 @@
   import { formatDuration } from '$lib/media/duration';
   import { FALLBACK_COVER, coverUrl } from '$lib/media/covers';
   import { dominantColorFromUrl, rgbToCssString } from '$lib/media/dominantColor';
+  import { useSmoothPosition } from '$lib/player/smoothPosition.svelte';
   import LikeButton from './LikeButton.svelte';
   import TrackMenu from './TrackMenu.svelte';
 
   const current = $derived(player.current);
 
+  // Per-frame interpolated playhead so the scrubber bar moves
+  // smoothly between server ticks instead of stepping every ~250ms.
+  // Reset on track/play-state change via the helper's $effect.
+  const smoothed = useSmoothPosition();
+
   const skipPrevDisabled = $derived(player.index === 0 && player.position < 3);
   const skipNextDisabled = $derived(
     player.index === player.queue.length - 1 && player.repeat === 'off'
@@ -226,10 +232,12 @@
         </div>
       </div>
 
-      <!-- Bottom row: full-width seek slider + time labels -->
+      <!-- Bottom row: full-width seek slider + time labels.
+           Reads the smoothed position so the slider thumb glides
+           between server ticks at playback rate. -->
       <div class="flex items-center gap-2">
         <span class="w-9 shrink-0 text-right text-[10px] tabular-nums text-text-secondary">
-          {formatDuration(player.position)}
+          {formatDuration(smoothed.value)}
         </span>
         <input
           type="range"
@@ -237,7 +245,7 @@
           min="0"
           max={player.duration || 0}
           step="0.1"
-          value={player.position}
+          value={smoothed.value}
           oninput={onSeekInput}
           class="flex-1 accent-accent"
         />
@@ -346,10 +354,11 @@
             <SkipForward size={20} strokeWidth={1.5} fill="currentColor" />
           </button>
         </div>
-        <!-- Seek row -->
+        <!-- Seek row — reads smoothed position so the thumb glides
+             between server ticks. See useSmoothPosition. -->
         <div class="flex items-center gap-3">
           <span class="w-12 shrink-0 text-right text-xs tabular-nums text-text-secondary">
-            {formatDuration(player.position)}
+            {formatDuration(smoothed.value)}
           </span>
           <input
             type="range"
@@ -357,7 +366,7 @@
             min="0"
             max={player.duration || 0}
             step="0.1"
-            value={player.position}
+            value={smoothed.value}
             oninput={onSeekInput}
             class="flex-1 accent-accent"
           />

web/src/lib/components/PlaylistCard.svelte

@@ -64,7 +64,15 @@
     starting = true;
     try {
       const variant = playlist.system_variant;
-      if (variant != null) {
+      // systemShuffle keys by variant alone, which only works for
+      // SINGLE-instance variants (for_you, discover, deep_cuts, …).
+      // songs_like_artist has one playlist PER seed artist; using
+      // the variant alone would return the wrong one (or fail).
+      // For those, fall through to getPlaylist so the play handler
+      // hits the exact playlist the user clicked on. Detect via
+      // seed_artist_id which is non-null only for per-artist mixes.
+      const isPerArtist = playlist.seed_artist_id != null;
+      if (variant != null && !isPerArtist) {
         // #415: server returns the rotation-aware order (unplayed
         // this rotation first). Play it as-is — no client shuffle —
         // and tag the queue so play_started carries `source` and
@@ -78,7 +86,10 @@
         const detail = await getPlaylist(playlist.id);
         const refs = toTrackRefs(detail.tracks);
         if (refs.length > 0) {
-          playQueue(refs, 0);
+          // Tag the queue with the variant when one exists so play
+          // events still carry the correct source attribution even
+          // though we went through the per-id path.
+          playQueue(refs, 0, variant != null ? { source: variant } : undefined);
         }
       }
     } finally {