Add progress bar to v3 "Up Next" pill (#649)

The persistent top-right "Up Next" pill showed the upcoming section name
and a countdown ("in 12.3s") but no at-a-glance sense of how far through
the current section the song is. Add a thin progress bar directly under
the existing text that fills as the current section elapses toward the
next, reaching full when the section flips.

The text row is wrapped unchanged in a flex row and the pill stacks the
bar beneath it; nothing else about the pill's content or styling changes.
Progress is computed in updateUpNext() as the fraction elapsed between the
previous section boundary (last section at/before now, else song start)
and the next section. The fill uses the same gradient as the section name
for visual cohesion.

Signed-off-by: topkoa <topkoa@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

static/v3/player-chrome.js

@@ -187,6 +187,19 @@
         const nm = $('v3-upnext-name'), eta = $('v3-upnext-eta');
         if (nm) nm.textContent = next.name || '—';
         if (eta) eta.textContent = 'in ' + dt.toFixed(1) + 's';
+        // Progress bar: fraction of the current section elapsed toward `next`.
+        // Previous boundary is the last section at/before now (else song start).
+        const fill = $('v3-upnext-bar-fill');
+        if (fill) {
+            let prevT = 0;
+            for (let i = 0; i < secs.length; i++) {
+                if (typeof secs[i].time === 'number' && secs[i].time <= t) prevT = secs[i].time;
+                else break;
+            }
+            const span = next.time - prevT;
+            const prog = span > 0 ? Math.max(0, Math.min(1, (t - prevT) / span)) : 0;
+            fill.style.width = (prog * 100).toFixed(1) + '%';
+        }
         pill.classList.remove('hidden');
     }