feat(slint): queue and lyrics buttons in the header

Add Queue and Lyrics icon buttons to the left of the hamburger. Visual
for now — they will toggle the right-side sidebar once it lands.

cc

crates/qbz-slint/ui/assets/icons/captions.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <rect width="18" height="14" x="3" y="5" rx="2" />
+  <path d="M7 15h4" />
+  <path d="M15 15h2" />
+  <path d="M7 11h2" />
+  <path d="M13 11h4" />
+</svg>

crates/qbz-slint/ui/assets/icons/list-music.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M21 15V6" />
+  <path d="M18.5 18a2.5 2.5 0 1 0 0-5 2.5 2.5 0 0 0 0 5Z" />
+  <path d="M12 12H3" />
+  <path d="M16 6H3" />
+  <path d="M12 18H3" />
+</svg>