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

packages/web/src/lib/http.ts

@@ -1,12 +1,8 @@
 import { fetch as tauriFetch } from "@tauri-apps/plugin-http";
+import { isTauri } from "@/lib/platform";
 
 // Route fetch through Rust (reqwest) when running in Tauri — bypasses WebView CORS enforcement.
-// Falls back to native browser fetch during plain web dev (`pnpm dev:web`).
-const isTauri = Boolean(
-	typeof window !== "undefined" &&
-		(window as unknown as Record<string, unknown>).__TAURI_INTERNALS__,
-);
-
-export const httpFetch: typeof globalThis.fetch = isTauri
+// Falls back to native browser fetch during plain web dev.
+export const httpFetch: typeof globalThis.fetch = isTauri()
 	? (tauriFetch as typeof globalThis.fetch)
 	: globalThis.fetch;