fix: smooth NP swipe-open animation; live playing highlight in song lists

Swipe animation:
- _resetDrag(true) now reads the overlay's current drag position and starts
  the CSS transition from there instead of resetting to translateY(100%) first.
  That reset was causing the visible stutter (overlay jumped off-screen then
  slid back up). Now the overlay continues seamlessly from wherever the finger
  released to translateY(0).

Playing highlight:
- makeSongItem: adds data-song-id attribute to every .song-item element.
- updatePlayerUI: calls _updatePlayingHighlights() after every track change.
  This toggles .playing on all visible song items in any open list/playlist/
  favorites view so the highlighted row always matches what's actually playing,
  without requiring the user to navigate away and back.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.js

@@ -221,8 +221,29 @@ window.addEventListener('load', () => {
     function _resetDrag(open) {
       playerBar.classList.remove('dragging');
       playerBar.style.transform = '';
-      if(npOverlay) { npOverlay.style.transition = ''; npOverlay.style.transform = ''; }
-      if(open) openNowPlaying();
+
+      if (!npOverlay) { if(open) openNowPlaying(); _sy=0; _dragging=false; return; }
+
+      if (open) {
+        // Continue animating from the current drag position instead of
+        // resetting to translateY(100%) first (which caused the stutter).
+        const m = npOverlay.style.transform.match(/translateY\(([^p]+)px\)/);
+        const curY = m ? parseFloat(m[1]) : window.innerHeight;
+        npOverlay.style.transition = 'none';
+        npOverlay.style.transform  = `translateY(${curY}px)`;
+        void npOverlay.offsetHeight; // flush layout so curY is the transition start
+        npOverlay.style.transition = '';
+        npOverlay.style.transform  = 'translateY(0)';
+        npOverlay.addEventListener('transitionend', () => {
+          npOverlay.style.transform = '';
+          npOverlay.style.transition = '';
+        }, { once: true });
+        openNowPlaying();
+      } else {
+        npOverlay.style.transition = '';
+        npOverlay.style.transform  = '';
+      }
+
       _sy=0; _dragging=false;
     }