refactor: replace hardcoded hex values with Tailwind semantic tokens

3,200+ hardcoded color values replaced with CSS variable-backed
Tailwind classes (bg-card, text-foreground, border-border, etc.).
Enables light mode via CSS variable swap. Only syntax highlighting
colors and intentional one-offs remain hardcoded (~15 values).

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

frontend/src/components/tree-editor/NodeEditorModal.tsx

@@ -85,7 +85,7 @@ export function NodeEditorModal({ node, onClose, isNewNode = false }: NodeEditor
   return (
     <Modal isOpen={true} onClose={onClose} title={getTitle()} size="lg" footer={footerContent} allowFullScreen={true}>
       {/* Node ID display */}
-      <div className="mb-4 text-xs text-[#848b9b]">
+      <div className="mb-4 text-xs text-muted-foreground">
         Node ID: <code className="rounded bg-accent px-1 py-0.5">{node.id}</code>
       </div>