Namespace the `theme` localStorage key (django.admin.theme) to avoid clashing with main project localStorage

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

Rename the localStorage key for the /admin page from `theme` to `django.admin.theme`

### Problem

We use the `theme` localStorage for our site, decomp.me. We expect the value to be a json string (e.g. `"auto"`, `"light"` or `"dark"`, etc). When visiting the `/admin` panel, our localStorage value is stomped on and instead `auto` is set (no quotes). We have worked around the fact that this is not JSON (and so we fallback to "auto"), but this still stomps on our users preferences.

### Request or proposal

proposal

### Additional Details

_No response_

### Implementation Suggestions

I have a branch with the change: https://github.com/django/django/compare/main...mkst:django:namespace-admin-theme-key?expand=1