fix(vortex-web): restore treemap header band so strips stop overlapping

The header bars were piling up on top of each other (and on leaf labels) at
the top of the map because `.paddingTop(18)` was being overridden: d3's
`.paddingOuter(1)` also sets the top pad and was called after it, collapsing the
band to 1px. Set `paddingTop` last so each container gets a real 18px band, and
lay out PAD_TOP taller with an upward shift so the root's own band doesn't leave
a top margin. Strips now sit in their own bands with no overlap.

Signed-off-by: Robert <robert@spiraldb.com>

Author: robert3005

vortex-web/docs/img/treemap-explorer-chunks-dark.png

@@ -185,13 +185,23 @@ export function BlockTreemap({
       .sum((d) => (d.children ? 0 : d.bytes))
       .sort((a, b) => (b.value ?? 0) - (a.value ?? 0));
     treemap<TreeNode>()
-      .size([size.w, size.h])
+      // Lay out PAD_TOP taller; we shift up by PAD_TOP below so the root's own
+      // header band sits off-screen and top-level fields start flush with y=0.
+      .size([size.w, size.h + PAD_TOP])
       .tile(treemapSquarify)
-      .paddingTop(PAD_TOP)
       .paddingInner(2)
       .paddingOuter(1)
+      // paddingTop MUST come after paddingOuter — paddingOuter also sets the top
+      // pad, so setting it last would collapse the header band to 1px and make
+      // nested strips pile up on top of each other.
+      .paddingTop(PAD_TOP)
       .round(true)(r);
-    return r.descendants() as RectNode[];
+    const desc = r.descendants() as RectNode[];
+    for (const n of desc) {
+      n.y0 -= PAD_TOP;
+      n.y1 -= PAD_TOP;
+    }
+    return desc;
   }, [tree, size]);
 
   const byId = useMemo(() => {