feat(web,flutter): #401 — now-playing highlight on TrackRow + PlaylistTrackRow

Cross-platform consistency: when a track is currently playing, its row
on the album / liked / history / search / playlist detail surfaces
gets the same accent-border + bg-lift treatment that QueueTrackRow
applies on the queue panel. Closes the inconsistency caught during
the #375 DRY audit (the queue row had a "you are here" indicator;
other track-row surfaces did not).

Web — TrackRow.svelte + PlaylistTrackRow.svelte:
  isCurrent = player.current?.id === track.id
  → border-l-2 border-l-accent bg-surface-hover when true.
  PlaylistTrackRow additionally gates on !isUnavailable so deleted
  rows never match a phantom playing id.

Flutter — TrackRow + _PlaylistTrackRow:
  TrackRow becomes a ConsumerWidget, watches mediaItemProvider, and
  wraps its InkWell in a Container whose BoxDecoration carries the
  fs.iron background + 2px fs.accent left border when current. Title
  text also shifts to fs.accent and FontWeight.w500. Same pattern for
  _PlaylistTrackRow.

No "Now playing" pill on these surfaces — the player bar already
names the track, so the accent band alone reads as enough cue.

For #401 / #356 umbrella.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/PlaylistTrackRow.svelte

@@ -5,6 +5,7 @@
   import TrackMenu from './TrackMenu.svelte';
   import type { PlaylistTrack } from '$lib/api/types';
   import { playlistTrackToRef } from '$lib/playlists/playlistTrackToRef';
+  import { player } from '$lib/player/store.svelte';
   import { offsetToDelta } from './queue-row-math';
 
   let {
@@ -50,12 +51,20 @@
   // the dragEnd handler skips by returning before onMove fires; visually
   // the user never sees the row move.
   const canDrag = $derived(isOwner && !isUnavailable);
+
+  // "Now playing" highlight: matches QueueTrackRow's treatment so the
+  // user can see which row is current without reading the player bar.
+  // Unavailable rows (trackId === null) never match.
+  const isCurrent = $derived(
+    !isUnavailable && player.current?.id === row.track_id,
+  );
 </script>
 
 <div
   role="listitem"
   class="flex items-center gap-3 px-3 py-2 text-sm transition-colors hover:bg-surface-hover
-         {isUnavailable ? 'text-text-muted' : 'text-text-primary'}"
+         {isUnavailable ? 'text-text-muted' : 'text-text-primary'}
+         {isCurrent ? 'border-l-2 border-l-accent bg-surface-hover' : ''}"
   use:draggable={{
     axis: 'y',
     bounds: 'parent',