fix: use the correct design token color for the health chart background (#8783)

This change swaps out the color of the health chart "unfilled" section
for `theme.palette.background.application`. This is the same color
that's used in the sketches, so it should apply better for dark mode.

Why? I noticed that the graph looks choppy in dark mode, so figured
I'd go and investigate. This update makes it look a lot smoother.

Light mode looks the same as before.

Before (notice the leaking light grey):

![image](https://github.com/user-attachments/assets/535ca24b-756b-460c-a7ab-78daf28d68ba)


After:

![image](https://github.com/user-attachments/assets/98d4c0de-bde3-4d10-9210-fdd0bf5dc572)

frontend/src/component/project/Project/ProjectStatus/ProjectHealth.tsx

@@ -118,7 +118,7 @@ export const ProjectHealth = () => {
                             cy='50'
                             r={ChartRadius}
                             fill='none'
-                            stroke={theme.palette.grey[300]}
+                            stroke={theme.palette.background.application}
                             strokeWidth={ChartStrokeWidth}
                             strokeDasharray={`${filledLength * circumference} ${gapLength * circumference}`}
                             strokeDashoffset={offset * circumference}