Make agent font size inherit the UI font size by default (#36306)

Ensures issues like #36242 and #36295 do not arise where users are
confused that the agent panel does not follow the default UI font size
whilst also keeping the possibility of customization. The agent font
size was matching the UI font size previously alredy, which makes it
easier to change it for most scenarios.

Also cleans up some related logic around modifying the font sizes.

Release Notes:

- The agent panel font size will now inherit the UI font size by default
if not set in your settings.

crates/theme/src/theme.rs

@@ -107,6 +107,8 @@ pub fn init(themes_to_load: LoadThemes, cx: &mut App) {
     let mut prev_buffer_font_size_settings =
         ThemeSettings::get_global(cx).buffer_font_size_settings();
     let mut prev_ui_font_size_settings = ThemeSettings::get_global(cx).ui_font_size_settings();
+    let mut prev_agent_font_size_settings =
+        ThemeSettings::get_global(cx).agent_font_size_settings();
     cx.observe_global::<SettingsStore>(move |cx| {
         let buffer_font_size_settings = ThemeSettings::get_global(cx).buffer_font_size_settings();
         if buffer_font_size_settings != prev_buffer_font_size_settings {
@@ -119,6 +121,12 @@ pub fn init(themes_to_load: LoadThemes, cx: &mut App) {
             prev_ui_font_size_settings = ui_font_size_settings;
             reset_ui_font_size(cx);
         }
+
+        let agent_font_size_settings = ThemeSettings::get_global(cx).agent_font_size_settings();
+        if agent_font_size_settings != prev_agent_font_size_settings {
+            prev_agent_font_size_settings = agent_font_size_settings;
+            reset_agent_font_size(cx);
+        }
     })
     .detach();
 }