test: add Node.js 25 SSR regression test for the window->globalThis alias

Adds a node-environment test that recreates the Node.js 25 environment where
`window` is aliased to `globalThis` and `localStorage.getItem` throws.
It asserts `ThemeProvider` renders to static markup without throwing and that
server rendering never reads from `localStorage`. The test fails on the old
`typeof window` guard and passes with the `typeof document` guard.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EduardF1

next-themes/__tests__/ssr-node25.test.tsx

@@ -0,0 +1,96 @@
+// @vitest-environment node
+
+import * as React from 'react'
+import { renderToStaticMarkup } from 'react-dom/server'
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest'
+
+// Regression test for https://github.com/pacocoursey/next-themes/issues/389
+//
+// Node.js 25 ships an experimental Web Storage API where `window` is aliased to
+// `globalThis`. That makes the historic `typeof window === 'undefined'` SSR guard
+// evaluate to `false` on the server, so next-themes wrongly takes the browser code
+// path during SSR and crashes (e.g. `localStorage.getItem is not a function` /
+// `window.matchMedia is not a function`).
+//
+// These tests recreate the Node 25 server environment and assert that server
+// rendering no longer touches browser-only globals.
+
+describe('Node.js 25 SSR guard (window aliased to globalThis)', () => {
+  const globals = globalThis as Record<string, unknown>
+  const hadWindow = 'window' in globals
+  const hadLocalStorage = 'localStorage' in globals
+  const originalWindow = globals.window
+  const originalLocalStorage = globals.localStorage
+
+  beforeEach(() => {
+    vi.resetModules()
+
+    // Node 25 aliases `window` to `globalThis` on the server.
+    globals.window = globalThis
+
+    // Node 25 exposes a *partial* localStorage. Simulate the broken state where
+    // calling `getItem` throws, matching the original crash report.
+    Object.defineProperty(globalThis, 'localStorage', {
+      configurable: true,
+      writable: true,
+      value: {
+        getItem() {
+          throw new TypeError('localStorage.getItem is not a function')
+        }
+      }
+    })
+
+    // `document` and `window.matchMedia` remain browser-only and are absent on the
+    // server even on Node 25.
+    expect(typeof (globalThis as { document?: unknown }).document).toBe('undefined')
+  })
+
+  afterEach(() => {
+    if (hadWindow) {
+      globals.window = originalWindow
+    } else {
+      delete globals.window
+    }
+
+    if (hadLocalStorage) {
+      Object.defineProperty(globalThis, 'localStorage', {
+        configurable: true,
+        writable: true,
+        value: originalLocalStorage
+      })
+    } else {
+      delete globals.localStorage
+    }
+
+    vi.resetModules()
+  })
+
+  test('ThemeProvider renders to static markup without throwing', async () => {
+    const { ThemeProvider } = await import('../src/index')
+
+    expect(() =>
+      renderToStaticMarkup(
+        React.createElement(ThemeProvider, null, React.createElement('span', null, 'child'))
+      )
+    ).not.toThrow()
+  })
+
+  test('server render does not read from the broken localStorage', async () => {
+    const getItem = vi.fn(() => {
+      throw new TypeError('localStorage.getItem is not a function')
+    })
+    Object.defineProperty(globalThis, 'localStorage', {
+      configurable: true,
+      writable: true,
+      value: { getItem }
+    })
+
+    const { ThemeProvider } = await import('../src/index')
+
+    renderToStaticMarkup(
+      React.createElement(ThemeProvider, { defaultTheme: 'system', enableSystem: true }, 'child')
+    )
+
+    expect(getItem).not.toHaveBeenCalled()
+  })
+})