[Code scan] Avoid mutating the global sitemap while rendering the banner

This issue is a result of a Codex global code scan of deepmodeling/deepmodeling_sphinx at commit 156679fe55fc61e8ec3b1e9be8a45fde3567b9cb.

Problem

`render_banner()` writes the active CSS class directly into the module-level `sitemap`. The mutation persists across subsequent calls, so rendering one site as active and then rendering another can leave multiple navigation items marked active.

Code references:

https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/config.py#L1-L33
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/inject.py#L16-L35

Reproduction

```py
from deepmodeling_sphinx.inject import render_banner
from deepmodeling_sphinx.config import sitemap

render_banner("Docs")
render_banner("Blog")
print([(item["title"], item.get("class")) for item in sitemap if item.get("class")])
```

The output includes both `Docs` and `Blog` with `active docs-active`.

Impact

Repeated in-process builds, tests, or custom integrations that render the banner with different `deepmodeling_current_site` values can leak active state between renders.

Suggested fix

Build a per-render copy of the sitemap and assign active state on that copy, or compute the class in the Jinja template without mutating shared module state.

Read deepmodeling_sphinx/config.py and deepmodeling_sphinx/inject.py, then run the issue's two-call render_banner reproduction. Ensure active state is isolated to each render so rendering Docs followed by Blog leaves only Blog active, without mutating the module-level sitemap.

Written by the indexing model from the issue text.