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.rs

@@ -515,6 +515,7 @@ pub struct GitPanelSettingsContent {
     /// Default width of the panel in pixels.
     ///
     /// Default: 360
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub default_width: Option<f32>,
     /// How entry statuses are displayed.
     ///
@@ -586,6 +587,7 @@ pub struct NotificationPanelSettingsContent {
     /// Default width of the panel in pixels.
     ///
     /// Default: 300
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub default_width: Option<f32>,
 }
 
@@ -603,6 +605,7 @@ pub struct PanelSettingsContent {
     /// Default width of the panel in pixels.
     ///
     /// Default: 240
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub default_width: Option<f32>,
 }
 
@@ -777,6 +780,7 @@ pub struct OutlinePanelSettingsContent {
     /// Customize default width (in pixels) taken by outline panel
     ///
     /// Default: 240
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub default_width: Option<f32>,
     /// The position of outline panel
     ///
@@ -797,6 +801,7 @@ pub struct OutlinePanelSettingsContent {
     /// Amount of indentation (in pixels) for nested items.
     ///
     /// Default: 20
+    #[serde(serialize_with = "crate::serialize_optional_f32_with_two_decimal_places")]
     pub indent_size: Option<f32>,
     /// Whether to reveal it in the outline panel automatically,
     /// when a corresponding project entry becomes active.