fix: replace typeof window with typeof document for SSR detection

Node.js 25+ ships window as an alias for globalThis (Web Storage API
experiment), which breaks the typeof window === 'undefined' SSR guard:
- isServer evaluates to false on the server
- ThemeScript nonce logic misbehaves server-side
- getTheme() calls localStorage.getItem() on the server
- Results in: TypeError: localStorage.getItem is not a function

Fix: use typeof document === 'undefined' instead. document is a
browser-only API that Node.js does not expose (even in v25+), making
it a reliable SSR guard across all Node.js versions.

Fixes #389

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

Author: EduardF1

next-themes/src/index.tsx

@@ -6,7 +6,7 @@ import type { Attribute, ThemeProviderProps, UseThemeProps } from './types'
 
 const colorSchemes = ['light', 'dark']
 const MEDIA = '(prefers-color-scheme: dark)'
-const isServer = typeof window === 'undefined'
+const isServer = typeof document === 'undefined'
 const ThemeContext = React.createContext<UseThemeProps | undefined>(undefined)
 const defaultContext: UseThemeProps = { setTheme: _ => { }, themes: [] }
 
@@ -212,7 +212,7 @@ export const ThemeScript = React.memo(
       <script
         {...scriptProps}
         suppressHydrationWarning
-        nonce={typeof window === 'undefined' ? nonce : ''}
+        nonce={typeof document === 'undefined' ? nonce : ''}
         dangerouslySetInnerHTML={{ __html: `(${script.toString()})(${scriptArgs})` }}
       />
     )