[tests] migrate from jest to vitest (#3940)

This is a big change, but I think it's a good move, as `vitest` is much
more modern than `jest`.

I'm excited about the UI watch feature (run `npm run test:ui`), for
example - it's really helpful and saves time when debugging tests. I had
to adjust a few tests because they had time related issues, but
basically we are now testing the same things - even a bit better and
less flaky (I hope).

What do you think?

vitest.config.mjs

@@ -0,0 +1,70 @@
+import {defineConfig} from "vitest/config";
+
+/*
+ * Sequential execution keeps our shared test server stable:
+ * - All suites bind to port 8080
+ * - Fixtures and temp paths are reused between tests
+ * - Debugging becomes predictable
+ *
+ * Parallel execution would require dynamic ports and isolated fixtures,
+ * so we intentionally cap Vitest at a single worker for now.
+ */
+
+export default defineConfig({
+	test: {
+		// Global settings
+		globals: true,
+		environment: "node",
+		// Setup files for require aliasing
+		setupFiles: ["./tests/utils/vitest-setup.js"],
+		// Increased from 20s to 60s for E2E tests, 120s for Electron tests
+		testTimeout: 120000,
+		// Increase hook timeout for Electron cleanup
+		hookTimeout: 30000,
+		// Stop test execution on first failure
+		bail: 1,
+
+		// File patterns
+		include: [
+			"tests/**/*_spec.js",
+			// Legacy regression test without the _spec suffix
+			"tests/unit/modules/default/calendar/calendar_fetcher_utils_bad_rrule.js"
+		],
+		exclude: [
+			"**/node_modules/**",
+			"**/dist/**",
+			"tests/unit/mocks/**",
+			"tests/unit/helpers/**",
+			"tests/electron/helpers/**",
+			"tests/e2e/helpers/**",
+			"tests/e2e/mocks/**",
+			"tests/configs/**",
+			"tests/utils/**"
+		],
+
+		// Coverage configuration
+		coverage: {
+			provider: "v8",
+			reporter: ["lcov", "text"],
+			include: [
+				"clientonly/**/*.js",
+				"js/**/*.js",
+				"modules/default/**/*.js",
+				"serveronly/**/*.js"
+			],
+			exclude: [
+				"**/node_modules/**",
+				"**/tests/**",
+				"**/dist/**"
+			]
+		},
+
+		/*
+		 * Pool settings for isolated test execution. Keep maxWorkers at 1 so
+		 * port 8080 and shared fixtures remain safe across the full suite.
+		 */
+		pool: "forks",
+		maxWorkers: 1,
+		isolate: true
+	}
+});