Adopt shared WebUI structural primitives

webui/scripts/test-dataflow-page-structure.mjs

@@ -60,11 +60,14 @@ const checks = [
       moduleEntry.indexOf("./styles/dataflow.css"),
     "XyFlow base styles before GovOPlaN overrides"
   ],
-  [css.includes("height: calc(100vh - 115px)"), "full-height workspace"],
+  [
+    page.includes('<WorkspaceFrame as="main" height="viewport"'),
+    "central full-height workspace"
+  ],
   [css.includes(".dataflow-preview-table-wrap"), "bounded preview scrolling"],
   [
-    /\.dataflow-palette-items\s*\{[^}]*grid-auto-rows:\s*max-content;[^}]*overflow-y:\s*auto;/s.test(css),
-    "bounded palette scrolling"
+    page.includes("<DefinitionPalette") && !css.includes(".dataflow-palette-items"),
+    "centralized bounded palette scrolling"
   ],
   [
     /\.dataflow-inspector-fields\s*\{[^}]*grid-auto-rows:\s*max-content;[^}]*overflow-y:\s*auto;/s.test(css),