Support configuring a custom background color for inline code spans via
the `style.inline_code.background` setting. This allows users to improve
the legibility of inline code when the theme's default background is too
subtle or clashes with the terminal's palette.

The `jp_md` formatter now accepts an optional hex color that, when
provided, overrides the theme-derived background for inline code.

In `jp_cli`, this configuration is passed from the global style config
to the markdown renderer during `query` and `conversation print`.

Example configuration in `config.toml`:

```toml
[style.inline_code]
background = "#3c3836"
```

Signed-off-by: Jean Mertz <git@jeanmertz.com>