fix(ui): make the queue button say whether the queue is open

It looked identical in both states, so the only way to tell what
pressing it would do was to look at the other side of the window and
infer it -- and for anyone not looking there was nothing to infer from:
no aria-expanded, no aria-controls, no drawn state.

The state is reflected *from the panel* rather than kept beside the
click. This button is not the only thing that opens the queue --
now-playing-view sets the same attribute, because it hides the bar the
button lives in -- so a flag maintained by the click handler would be
right until something else opened the panel and then quietly wrong.
The panel's `open` attribute stays the one fact; a MutationObserver
reflects it.

Refs #26

frontend/index.css

@@ -207,6 +207,17 @@ body div.sidebar {
         color: var(--yj-accent, #ffd43b);
     }
 
+    /* An open queue is a state this button can be in, and it used to
+       look exactly like the closed one -- so the only way to tell what
+       pressing it would do was to look at the other side of the window
+       and infer it. `aria-expanded` is the same fact for anyone not
+       looking at all, and it points at the panel it controls. */
+    #queue-button[aria-expanded='true'] {
+        color: var(--yj-accent, #ffd43b);
+        background: var(--yj-bg-overlay, #404040);
+        border-radius: 4px;
+    }
+
     #queue-button.drag-over {
         color: var(--yj-accent, #ffd43b);
         outline: 2px dashed var(--yj-accent, #ffd43b);