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/lib/devGate.ts

@@ -0,0 +1,12 @@
+/* Dev-only feature gate.
+ *
+ * Reads VITE_DECNET_DEVELOPER at build time. Vite inlines the value
+ * at compile, so a prod build with the flag unset becomes a constant
+ * `false` and the route guard plus its lazy import are tree-shaken
+ * out of the bundle entirely.
+ *
+ * Set in .env.development:  VITE_DECNET_DEVELOPER=1
+ */
+export function isDeveloperMode(): boolean {
+  return import.meta.env.VITE_DECNET_DEVELOPER === '1';
+}