useTheme throws during SSR when localStorageKey is configured #2974

Description

useTheme({ localStorageKey: 'theme' }) throws during server-side rendering when browser localStorage is unavailable.

The hook guards window.matchMedia with isBrowser, but its theme-mode initializer still accesses localStorage.getItem whenever localStorageKey is non-empty. This remains present on current master (43003f3bd5e6921173cfa2a791ed9d34e5221935, package version 3.10.1). The earlier SSR fix #2691 protected the media query path, but not this storage path.

Minimal reproduction

Run in a server environment without browser storage:

import { renderToString } from 'react-dom/server';
import { useTheme } from 'ahooks';

function App() {
  const { theme, themeMode } = useTheme({ localStorageKey: 'theme' });
  return <span>{themeMode}:{theme}</span>;
}

renderToString(<App />);

Expected behavior

Rendering succeeds using the existing server defaults (themeMode: 'system', theme: 'light'). Browser rendering continues to read and persist the selected mode.

Actual behavior

The initializer accesses unavailable localStorage. The regression test with localStorage explicitly unavailable throws TypeError: Cannot read properties of undefined (reading 'getItem'); runtimes without that global can throw ReferenceError instead.

A server runtime that exposes its own localStorage also causes the hook to read that storage rather than using browser-only persistence.

Proposed fix and verification

Guard storage reads and writes with the existing isBrowser utility, following the repository's useLocalStorageState pattern.

Node-environment renderToString regressions fail on the original source and pass with the guard. Browser tests verify persisted modes, normal writes, empty/omitted keys, and stable setter references. A PR with the implementation and tests will follow.