Objective:

Allow users to configure the font families used by agent panel content
separately from the main UI and buffer fonts.

Solution:

- Add `agent_ui_font_family` for agent responses in the agent panel.
- Add `agent_buffer_font_family` for the agent panel message editor and
user messages.
- Keep context menus on the primary UI font family.
- Document the new settings and expose them in the settings UI.

Testing:

- Ran `cargo fmt`.
- Ran `cargo check -p settings_content -p theme_settings -p markdown -p
agent_ui -p settings_ui -p ui`.
- Ran `git diff --check`.
- Manually tested the agent panel font behavior in a dev build.

Release Notes:

- Added settings for configuring agent panel UI and buffer font
families.

Co-authored-by: Finn Evers <finn@zed.dev>