Code uses system document font instead of monospace font by default

### What Happened?

in #1670 Code was changed to use the document font instead of the monospace font.

### Steps to Reproduce

1. Open preferences → Interface
2. See that system font is Open Sans, not Roboto Mono

### Expected Behavior

Document font is for apps like Evince. Monospace font is what Code should use.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_

Start by reviewing the change referenced in issue #1670 and the Preferences → Interface font setting. Confirm where the default document or monospace font is selected, then verify that Code opens with the monospace font by default while the preference remains usable.

索引モデルが issue の本文から書いたものです。