fix(slint): stabilize album card overlay buttons

The overlay buttons were outline-only with a transparent center, so the
pointer flickered between the ring and the hollow middle, and the
flicker made the more button's menu impossible to trigger. The buttons
now have a solid (semi-transparent) fill, the play glyph is filled, and
the pointer-gating feedback loop is gone. The overlay stays visible
while hovering a button (the states are OR-combined, since a child
button steals has-hover from the artwork detector).

cc

crates/qbz-slint/ui/assets/icons/play-fill.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#ffffff" stroke="none">
+  <path d="M8 5v14l11-7z" />
+</svg>