config: return ManimColor instance from background_color property

Nobody has claimed this yet.

## Source

`manim/_config/utils.py`, line 1193 (as of HEAD ):

```
# TODO: This was parsed before maybe add ManimColor(val), but results in circular import
@property
def background_color(self) -> ManimColor:
"""Background color of the scene (-c)."""
return self._d["background_color"]

```

## Problem

The `background_color` property is type-annotated as `ManimColor`, but the underlying value stored in `self._d` is not wrapped in a `ManimColor` instance. The natural fix, `return ManimColor(self._d["background_color"])`, is blocked by a circular import between `_config/utils.py` and the `ManimColor` definition.

The mismatch between the annotation and the runtime value can mislead callers that rely on `ManimColor` methods being available on the returned object.

## Suggested approaches

- Lazy import of `ManimColor` inside the getter.
- Restructure the import graph so `_config/utils.py` can import `ManimColor` at module level.
- Audit other config properties that may have the same gap (e.g. other color-typed config values).

Start in manim/_config/utils.py at line 1193 and trace the import relationship to the ManimColor definition. Compare the getter's annotation with the stored value and inspect related color-typed config properties. Done means background_color returns a ManimColor instance without introducing a circular import, with coverage for the corrected behavior.

Written by the indexing model from the issue text.