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.
