feat(decnet_web/theme-lab): scaffold dev-gated /theme-lab route

Adds VITE_DECNET_DEVELOPER build-time gate: when unset, the
isDeveloperMode() helper collapses to a constant false and Vite
tree-shakes both the lazy import and the conditional <Route> out
of the prod bundle.

ThemeLab is currently a header stub; subsequent tasks fill it
with the design-system primitive zoo plus a Dark/Light toggle
for live token tuning. Route is intentionally absent from
ROUTE_LABELS / sidebar — direct URL only.

decnet_web/src/components/ThemeLab/ThemeLab.css

@@ -0,0 +1,21 @@
+/* ThemeLab — lab-page-specific layout only.
+ * Colours come from index.css tokens; no hex literals here. */
+
+.theme-lab {
+  padding: var(--space-8);
+  color: var(--text-color);
+  font-family: var(--font-mono);
+}
+
+.theme-lab .page-header h1 {
+  font-size: var(--fs-page);
+  letter-spacing: var(--ls-title);
+  color: var(--accent-color);
+  margin-bottom: var(--space-2);
+}
+
+.theme-lab-subtitle {
+  font-size: var(--fs-mini);
+  letter-spacing: var(--ls-label);
+  opacity: 0.6;
+}

decnet_web/src/components/ThemeLab/ThemeLab.tsx

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+import React from 'react';
+import './ThemeLab.css';
+
+/* Kitchen-sink theme lab.
+ *
+ * Dev-only page (gated upstream in App.tsx via isDeveloperMode()).
+ * Subsequent tasks fill this in with every design-system primitive
+ * and a Dark/Light toggle. For now: header stub so the route + gate
+ * can land in isolation. */
+const ThemeLab: React.FC = () => {
+  return (
+    <div className="theme-lab" data-testid="theme-lab">
+      <header className="page-header">
+        <h1>THEME LAB</h1>
+        <p className="theme-lab-subtitle">
+          dev only · primitive zoo for theme regression
+        </p>
+      </header>
+    </div>
+  );
+};
+
+export default ThemeLab;

decnet_web/src/components/ThemeLab/__tests__/ThemeLab.test.tsx

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+import { describe, it, expect } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import ThemeLab from '../ThemeLab';
+
+describe('ThemeLab', () => {
+  it('renders the page header stub', () => {
+    render(<ThemeLab />);
+    expect(screen.getByTestId('theme-lab')).toBeInTheDocument();
+    expect(screen.getByText(/THEME LAB/i)).toBeInTheDocument();
+    expect(screen.getByText(/dev only/i)).toBeInTheDocument();
+  });
+});