refactor: migrate remaining components to Design System v4

111 files across 14 directories: common, tree-editor, kb-accelerator,
copilot, assistant, analytics, library, procedural, procedural-editor,
public, script-editor, ui, admin, step-library.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/common/PageLoader.tsx

@@ -5,7 +5,7 @@ export function PageLoader() {
     <div className="flex h-full items-center justify-center">
       <div className="flex flex-col items-center gap-4">
         <Spinner size="lg" />
-        <p className="text-sm text-muted-foreground">Loading&hellip;</p>
+        <p className="text-sm text-[#848b9b]">Loading&hellip;</p>
       </div>
     </div>
   )