refactor(web): extract isTauri into a leaf platform module

packages/web/src/lib/discovery.ts

@@ -1,14 +1,13 @@
 import { httpFetch } from "@/lib/http";
+import { isTauri } from "@/lib/platform";
+
+export { isTauri } from "@/lib/platform";
 
 export interface DiscoveredInstance {
 	port: number;
 	base_url: string;
 }
 
-export function isTauri(): boolean {
-	return typeof window !== "undefined" && "__TAURI_INTERNALS__" in window;
-}
-
 /**
  * Probe localhost ports for running Honcho instances. Desktop-only — the web
  * build can't port-scan due to CORS, so this returns an empty list when not