fix(slint): jump-nav natural-top at body-row top, no header overlap

The previous binding pulled the bar 44px up from body-row's top
(`body-row.y - self.height`), which landed it inside the artist
header — eating the action buttons row. The right value is
`body-row.y` itself: the bar takes the top 44px of the 52px
reservation slot that lives as body-row's first child, so it sits
exactly in the gap between the header above and the body below
(Popular Tracks header starts 52px into the body row, giving an
8px breathing gap below the bar). No overlap on either side.

Sidebar natural-top compensates with `+ jump-nav.height` so it
still starts flush with the bar's bottom edge as before.

cc

crates/qbz-slint/ui/artist/ArtistPageView.slint

@@ -684,14 +684,15 @@ export component ArtistPageView inherits Rectangle {
     // y is the row just below the artist header in the page layout
     // (NavButtons + spacer + artist header + small gap, ~290px).
     jump-nav := JumpNavBar {
-        // Dynamic anchor — sits 44px above the body row, regardless
-        // of how tall the artist header grew. As the user scrolls
-        // past the header, the bar pins to viewport top
-        // (sticky-top = 0).
+        // Sits AT the body row's top, taking the 44 top pixels of
+        // the 52px reservation slot the body row's first child
+        // carves out — so the bar never overlaps the artist header
+        // above (header ends exactly at body-row.y) and never
+        // overlaps Popular Tracks below (Popular Tracks header
+        // starts 52px into the body row, an 8px gap below the bar).
         property <length> natural-top:
             body-row.absolute-position.y
-            - page-flickable.absolute-position.y
-            - self.height;
+            - page-flickable.absolute-position.y;
         property <length> sticky-top: 0px;
 
         x: 0;
@@ -727,13 +728,13 @@ export component ArtistPageView inherits Rectangle {
         // bottom edge is always flush with the now-playing bar
         // (height = root.height - y), so no gap appears when sticky.
         if NetworkSidebarState.open: sidebar-panel := Rectangle {
-            // Anchored to the body row's actual top, so the sidebar
-            // always starts flush with the bottom of the JUMP TO bar
-            // (which itself sits 44px above the body row). Matches
-            // long-bio artist pages where the header pushes
-            // everything further down.
+            // Anchored 44px below the body row's top (= bottom edge
+            // of the JUMP TO bar, which itself starts AT body row
+            // top). The +44 ensures the sidebar always begins
+            // flush with the bar's bottom edge, no overlap.
             property <length> natural-top:
-                body-row.absolute-position.y - page-flickable.absolute-position.y;
+                body-row.absolute-position.y - page-flickable.absolute-position.y
+                + jump-nav.height;
             // 44px parks the sidebar just below the sticky JUMP TO
             // bar (jump-nav's height) once the header scrolls away.
             property <length> sticky-top: 44px;