fix(vortex-web): make stuck treemap header bars opaque

A pinned (stuck) sticky header bar was translucent, so content scrolling
underneath bled through it. Draw an opaque surface base beneath the dtype tint
once a bar is stuck, so it reads as solid and the children sit cleanly below
it. In-band (non-stuck) strips keep their light tint.

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

Author: robert3005

vortex-web/docs/img/treemap-explorer-hover.png

@@ -96,6 +96,8 @@ interface ThemeColors {
   dim: string;
   border: string;
   highlight: string;
+  /** Opaque page surface — used as the solid base of a stuck header bar. */
+  surface: string;
 }
 
 function resolveThemeColors(choice: 'light' | 'dark' | 'system'): ThemeColors {
@@ -104,8 +106,20 @@ function resolveThemeColors(choice: 'light' | 'dark' | 'system'): ThemeColors {
       ? window.matchMedia('(prefers-color-scheme: dark)').matches
       : choice === 'dark';
   return isDark
-    ? { fg: '#e4e4e8', dim: '#71717a', border: 'rgba(255,255,255,0.12)', highlight: '#2CB9D1' }
-    : { fg: '#18181b', dim: '#71717a', border: 'rgba(0,0,0,0.1)', highlight: '#2CB9D1' };
+    ? {
+        fg: '#e4e4e8',
+        dim: '#71717a',
+        border: 'rgba(255,255,255,0.12)',
+        highlight: '#2CB9D1',
+        surface: '#1a1a1e',
+      }
+    : {
+        fg: '#18181b',
+        dim: '#71717a',
+        border: 'rgba(0,0,0,0.1)',
+        highlight: '#2CB9D1',
+        surface: '#fafafa',
+      };
 }
 
 interface Transform {
@@ -482,6 +496,11 @@ export function BlockTreemap({
             const label = maxChars < 2 ? '' : truncate(h.name, maxChars);
             return (
               <g key={`hdr-${h.id}`} pointerEvents="none">
+                {/* Opaque base when stuck so children scrolling underneath are
+                    hidden — the bar reads as solid and pushes content below it. */}
+                {h.stuck && (
+                  <rect x={h.sx} y={h.top} width={h.sw} height={HEADER_H} fill={theme.surface} />
+                )}
                 <rect
                   x={h.sx}
                   y={h.top}