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/discover/HomeView.slint

@@ -149,7 +149,12 @@ export component HomeView inherits Rectangle {
             // overlap the right controls.
             left-controls := HorizontalLayout {
                 x: 32px;
-                y: (parent.height - self.height) / 2;
+                // Align the toolbar content's vertical center to y=25 —
+                // 11px (the back/forward padding-top used by the album /
+                // artist / settings views) + 14px (half the 28px nav
+                // buttons). Keeps the [<] [>] pair at the same screen
+                // position across every view, with no jump on navigation.
+                y: 25px - self.height / 2;
                 width: self.preferred-width;
                 // Explicit height too — without it the layout fills the
                 // 56px toolbar, so `y` resolves to 0 and the tab pill
@@ -181,7 +186,8 @@ export component HomeView inherits Rectangle {
             // on top of the history arrows.
             right-controls := HorizontalLayout {
                 x: parent.width - self.width - 32px;
-                y: (parent.height - self.height) / 2;
+                // Same y baseline as the left controls (see above).
+                y: 25px - self.height / 2;
                 width: self.preferred-width;
                 height: self.preferred-height;
                 spacing: 6px;

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

@@ -217,8 +217,10 @@ component HeaderIconButton inherits Rectangle {
         source: root.icon;
         width: 19px;
         height: 19px;
-        x: (parent.width - self.width) / 2;
-        y: (parent.height - self.height) / 2;
+        // Round the centering to whole pixels — a fractional offset
+        // (here 8.5px) makes the glyph render blurry.
+        x: Math.round((parent.width - self.width) / 2 / 1px) * 1px;
+        y: Math.round((parent.height - self.height) / 2 / 1px) * 1px;
         tint: root.active ? Theme.accent
             : ta.has-hover ? Theme.text-primary : Theme.text-secondary;
     }
@@ -248,7 +250,7 @@ export component HeaderBar inherits Rectangle {
         x: Spacing.md;
         y: (root.height - self.height) / 2;
         height: 30px;
-        spacing: Spacing.xs;
+        spacing: 2px;
         VerticalLayout {
             alignment: center;
             SidebarToggle { }

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