chore: finish frontend ESLint migration #18

Enable every plugin recommended preset and replace Biome with ESLint formatting.

Preserve API contracts and JSX whitespace; verify source, tests, builds and tooling.

src/test/setup.ts

@@ -1,28 +1,30 @@
 import "@testing-library/jest-dom/vitest";
 import { cleanup, configure } from "@testing-library/react";
-import { afterAll, afterEach, beforeAll } from "vitest";
+import { afterAll, afterEach, beforeAll, vi } from "vitest";
+
 import { configureAuthClient } from "@/shared/api";
+
 import "../shared/i18n";
 import { server } from "./server";
 
 // Allow async UI updates on the slower CI runner; local waits stay unchanged.
-if (process.env.CI) configure({ asyncUtilTimeout: 5_000 });
+if (process.env.CI) configure({ asyncUtilTimeout: 5000 });
 
 beforeAll(() => server.listen({ onUnhandledRequest: "error" }));
 afterEach(() => {
 	cleanup();
-	configureAuthClient(null);
+	configureAuthClient();
 	server.resetHandlers();
 });
 afterAll(() => server.close());
 
 class ResizeObserver {
+	disconnect() {}
 	observe() {}
 	unobserve() {}
-	disconnect() {}
 }
 
-window.ResizeObserver = ResizeObserver;
+vi.stubGlobal("ResizeObserver", ResizeObserver);
 
 // JSDOM has no layout/scrolling; Mantine Combobox scrolls the active option.
 Object.defineProperty(HTMLElement.prototype, "scrollIntoView", {
@@ -30,16 +32,16 @@ Object.defineProperty(HTMLElement.prototype, "scrollIntoView", {
 	value: () => {},
 });
 
-Object.defineProperty(window, "matchMedia", {
-	writable: true,
+Object.defineProperty(globalThis, "matchMedia", {
 	value: (query: string) => ({
+		addEventListener: () => {},
+		addListener: () => {},
+		dispatchEvent: () => false,
 		matches: false,
 		media: query,
-		onchange: null,
-		addEventListener: () => {},
+		onchange: /* eslint-disable-line unicorn/no-null -- Exercise the explicit null returned by the production API/DOM contract. */ null,
 		removeEventListener: () => {},
-		addListener: () => {},
 		removeListener: () => {},
-		dispatchEvent: () => false,
 	}),
+	writable: true,
 });