fix(palette-picker): swizzle NavbarItem/index instead of ComponentTypes

ComponentTypes swizzle was silently not loading. Replacing it with a
NavbarItem/index wrapper that intercepts custom-PalettePicker before
the ComponentTypes lookup, which is more reliable across Docusaurus
theme chain configurations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sserrata

demo/src/theme/NavbarItem/ComponentTypes.tsx

@@ -0,0 +1,23 @@
+/* ============================================================================
+ * NavbarItem swizzle — intercepts custom-PalettePicker before
+ * ComponentTypes lookup so no registry change is needed.
+ * ========================================================================== */
+
+import React from "react";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+import OriginalNavbarItem from "@theme-original/NavbarItem";
+
+import PalettePicker from "@site/src/components/PalettePicker";
+
+type OriginalProps = React.ComponentProps<typeof OriginalNavbarItem>;
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export default function NavbarItem(props: OriginalProps): JSX.Element {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  if ((props as any).type === "custom-PalettePicker") {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    return <PalettePicker mobile={(props as any).mobile} />;
+  }
+  return <OriginalNavbarItem {...props} />;
+}