[Bug]: typeof window comparison fails in NotFound component during SSR

### Consisely describe the issue

In `src/theme/NotFound.tsx`, there is an incorrect `typeof` comparison on line 14:
`if (typeof window !== undefined) {`
Because `typeof window` always evaluates to a string (either `"object"` or `"undefined"`), comparing it to the primitive `undefined` is always true.

If the project is ever built or rendered in a Server-Side Rendering (SSR) context where `window` is not defined, the current logic would throw a `ReferenceError: window is not defined` because it attempts to access `window.location.href`.

The comparison should check against the string `'undefined'`

### Upload screenshots or screen recordings

_No response_

### LLM Policy

- [x] This issue is human-written, and folow up comments will also be human-written

Open src/theme/NotFound.tsx and inspect the typeof window comparison on line 14, then trace how window.location.href is reached. Verify the corrected guard in an SSR context where window is unavailable; done means the component no longer attempts to access window during server rendering.

Written by the indexing model from the issue text.