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

packages/web/src/test/platform.test.ts

@@ -0,0 +1,17 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { isTauri } from "@/lib/platform";
+
+describe("isTauri", () => {
+	afterEach(() => {
+		delete (window as unknown as Record<string, unknown>).__TAURI_INTERNALS__;
+	});
+
+	it("returns false in a plain browser/jsdom environment", () => {
+		expect(isTauri()).toBe(false);
+	});
+
+	it("returns true when the Tauri internals global is present", () => {
+		(window as unknown as Record<string, unknown>).__TAURI_INTERNALS__ = {};
+		expect(isTauri()).toBe(true);
+	});
+});