Add agent panel font family settings (#59629)

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>

docs/src/visual-customization.md

@@ -66,9 +66,14 @@ If you would like to use distinct themes for light mode/dark mode that can be se
     "line_height": "standard",
   },
 
-  // Controls the font size for agent responses in the agent panel.
-  // If not specified, it falls back to the UI font size.
+  // Controls the font family and size for agent responses in the agent panel.
+  // If not specified, they fall back to the UI font family and size.
+  "agent_ui_font_family": "Inter",
   "agent_ui_font_size": 15,
+  // Controls the font family and size for the agent panel's message editor
+  // and user messages.
+  // If not specified, the font family falls back to the buffer font family.
+  "agent_buffer_font_family": "Berkeley Mono",
   // Controls the font size for the agent panel's message editor, user message,
   // and any other snippet of code.
   "agent_buffer_font_size": 12,
@@ -514,11 +519,14 @@ Project panel can be shown/hidden with {#action project_panel::ToggleFocus} ({#k
     "default_width": 640, // Default width (left/right docked)
     "default_height": 320 // Default height (bottom docked)
   },
-  // Controls the font size for agent responses in the agent panel.
-  // If not specified, it falls back to the UI font size.
+  // Controls the font family and size for agent responses in the agent panel.
+  // If not specified, they fall back to the UI font family and size.
+  "agent_ui_font_family": "Inter",
   "agent_ui_font_size": 15,
-  // Controls the font size for the agent panel's message editor, user message,
-  // and any other snippet of code.
+  // Controls the font family and size for the agent panel's message editor
+  // and user messages.
+  // If not specified, the font family falls back to the buffer font family.
+  "agent_buffer_font_family": "Berkeley Mono",
   "agent_buffer_font_size": 12
 }
 ```