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/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 {