show progress bar before waveform loads

- Draw progress bar track background unconditionally when no waveform
  exists, so the waveform area never appears blank
- Call drawWaveform() from onLoadedmetadata() so the bar fill appears
  as soon as metadata is available

index.html

@@ -676,13 +676,13 @@ function drawWaveform() {
   ctx.clearRect(0, 0, visWidth, h);
   if (!t) return;
   if (!t.waveform) {
-    if (audio && audio.duration) {
-      const pct = audio.currentTime / audio.duration;
     const barH = 4, y = (h - barH) / 2;
     ctx.fillStyle = '#272c42';
     ctx.beginPath();
     ctx.roundRect(0, y, visWidth, barH, 2);
     ctx.fill();
+    if (audio && audio.duration) {
+      const pct = audio.currentTime / audio.duration;
       ctx.fillStyle = '#84a0c6';
       ctx.beginPath();
       ctx.roundRect(0, y, visWidth * pct, barH, 2);
@@ -1562,6 +1562,7 @@ function onLoadedmetadata(e) {
   if (t && audio.duration) {
     t.playtime = audio.duration;
     renderTracks();
+    drawWaveform();
     computeWaveform(t);
   }
 }