fix(a11y): mark the playing row with a shape, not only a colour

a11y.22, WCAG 1.4.1: `.track-row.active` was a background tint and a
text colour, and the row markup carried no aria-current either — so a
colour-blind user could not find the playing row and AT had no signal
at all. The queue panel had aria-current from Phase 1 and the same
colour-only visual.

A triangle drawn in each row's own left padding by `::before`. It is a
shape that is present or absent, and it costs no layout: track-list's
grid columns are computed from the host width, so a marker in the flow
would move every cell on the playing row and nothing else.

Both directions are asserted in both tiers. A marker that renders on
every row satisfies "the playing row has one" for free, which is this
plan's oldest rule.

And one thing the reproduction found: a track started from the *list*
leaves the queue's currentIndex at -1, so the panel has no current row
in that flow at all. Pre-existing, and the reason this looked broken
the first time it was checked in the running app.

frontend/src/components/queue-panel/queue-panel.ts

@@ -378,6 +378,22 @@ export class QueuePanel
             color: var(--yj-accent-text, #ffd43b);
         }
 
+        /* a11y.22, the same rule as track-list one panel over: the
+           playing row was a tint and a text colour and nothing else.
+           The triangle lives in the row's own 16px left padding, so it
+           is a shape that is present or absent and the flex row does
+           not move. */
+        .track-item.active::before {
+            content: '';
+            position: absolute;
+            left: 5px;
+            top: 50%;
+            transform: translateY(-50%);
+            border-left: 5px solid var(--yj-accent-text, #ffd43b);
+            border-top: 4px solid transparent;
+            border-bottom: 4px solid transparent;
+        }
+
         .track-art {
             width: 32px;
             height: 32px;