fix(slint): crisp button icons, tighter tabs, homologate nav buttons

Icon buttons (header lyrics/queue/hamburger, queue footer) centered
their glyph at a fractional offset (e.g. (36-19)/2 = 8.5px), which
renders blurry — the offset is now rounded to whole pixels. The
header tab spacing dropped from 4px to 2px. The Discover toolbar's
back/forward pair drifted 3px from where the album/artist/settings
views place it; both toolbar clusters now anchor their vertical
center to y=25 so the [<] [>] pair holds one position across views.

cc

crates/qbz-slint/ui/shell/QueueSidebar.slint

@@ -79,8 +79,9 @@ component IconButton inherits Rectangle {
         source: root.icon;
         width: root.icon-size;
         height: root.icon-size;
-        x: (parent.width - self.width) / 2;
-        y: (parent.height - self.height) / 2;
+        // Round to whole pixels — a fractional offset renders blurry.
+        x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+        y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
         tint: !root.enabled
             ? Theme.text-disabled
             : (root.active