fix(furni): keep classname env read out of the browser bundle

`src/lib/furni/classname.ts` is imported by client components for its pure
string helpers, but it read `process.env.FURNI_BUNDLE_EXT` at module load.
In any client bundle without a `process` shim — including the UI fixture's
esbuild build — that threw `ReferenceError: process is not defined`, so the
lazily-loaded organize-imports dialog never mounted and `pnpm test:ui`
failed (studio.spec.ts). Fall back to the default extension when `process`
is absent; server behaviour is unchanged.

src/lib/furni/classname.ts

@@ -28,6 +28,10 @@ export type FurnitureBundleExt = (typeof BUNDLE_EXT_CHOICES)[number];
  */
 function readBundleExtEnv(): string | undefined {
 	const name = "FURNI_BUNDLE_EXT";
+	// `classname.ts` is also imported by client components for its pure string
+	// helpers, where `process` does not exist at runtime. Reading env is a
+	// server concern; on the client fall back to the default extension.
+	if (typeof process === "undefined") return undefined;
 	return process.env[name]?.trim() || undefined;
 }