settings_ui: Fix IEEE 754 floating point error when serializing value to JSON (#40677)

Closes #40556

Release Notes:

- settings-ui: Fixed an issue where modifying floating point number fields could result in the values written to the settings file containing IEEE 754 floating point error

> [!Note]
> Seems like there's another pull request fixing the centered layout in
#40661 by normalizing the whole `settings.json` file when updating it.
Not sure which is the better way to handle this issue.

## Description

This pull request solves the IEEE 754 floating point error when
serializing values from settings-ui into `settings.json` by creating a
two `serde_helper` to convert the problematic f32 into a formatted two
decimal placed f32.

Fields currently exists the IEEE 754 error:
- Appearance → Unnecessary Code Fade
- Editor → Drop Size Target
- Window & Layout → Centered Layout Left/Right Padding
- Window & Layout → Inactive Opacity

## How to verify

### Unnecessary Code Fade

As Is | To Be
--- | ---
<video
src="https://github.com/user-attachments/assets/1bf4bad2-63c5-4b03-ac29-8b6b59569e16"
/> | <video
src="https://github.com/user-attachments/assets/dadcd4a1-651b-43dd-913f-edae073ceb68"
/>

### Drop Size Target

As Is | To Be
--- | ---
<video
src="https://github.com/user-attachments/assets/9d5b4173-fcac-44d0-b7fc-772a2e426ef1"
/> | <video
src="https://github.com/user-attachments/assets/4b5adeaf-e678-494d-bd1b-6c1d55824c43"
/>

### Centered Layout Left/Right Padding

As Is | To Be
--- | ---
<video
src="https://github.com/user-attachments/assets/33b4e1ff-7ab2-44f7-9e9b-8abad1565d9a"
/> | <video
src="https://github.com/user-attachments/assets/63d8de9e-28d1-4bd7-a6c9-02452e105486"
/>

### Inactive Opacity

As Is | To Be
--- | ---
<video
src="https://github.com/user-attachments/assets/a7fe2e72-deb6-41dc-82f3-e2649503b8a4"
/> | <video
src="https://github.com/user-attachments/assets/993c314f-b6f6-4dcd-8f74-fa357ab063e9"
/>

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>

crates/settings/src/settings_content/theme.rs

@@ -9,6 +9,8 @@ use std::{fmt::Display, sync::Arc};
 
 use serde_with::skip_serializing_none;
 
+use crate::serialize_f32_with_two_decimal_places;
+
 /// Settings for rendering text in UI and text buffers.
 
 #[skip_serializing_none]
@@ -16,6 +18,7 @@ use serde_with::skip_serializing_none;
 pub struct ThemeSettingsContent {
     /// The default font size for text in the UI.
     #[serde(default)]
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub ui_font_size: Option<f32>,
     /// The name of a font to use for rendering in the UI.
     #[serde(default)]
@@ -42,6 +45,7 @@ pub struct ThemeSettingsContent {
     pub buffer_font_fallbacks: Option<Vec<FontFamilyName>>,
     /// The default font size for rendering in text buffers.
     #[serde(default)]
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub buffer_font_size: Option<f32>,
     /// The weight of the editor font in CSS units from 100 to 900.
     #[serde(default)]
@@ -56,9 +60,11 @@ pub struct ThemeSettingsContent {
     pub buffer_font_features: Option<FontFeatures>,
     /// The font size for agent responses in the agent panel. Falls back to the UI font size if unset.
     #[serde(default)]
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub agent_ui_font_size: Option<f32>,
     /// The font size for user messages in the agent panel.
     #[serde(default)]
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub agent_buffer_font_size: Option<f32>,
     /// The name of the Zed theme to use.
     #[serde(default)]
@@ -104,7 +110,7 @@ pub struct ThemeSettingsContent {
     derive_more::FromStr,
 )]
 #[serde(transparent)]
-pub struct CodeFade(pub f32);
+pub struct CodeFade(#[serde(serialize_with = "serialize_f32_with_two_decimal_places")] pub f32);
 
 impl Display for CodeFade {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {