fix(menu): alignment of projects and subprojects in sidebar (#1974)

This PR changes the width of the collapse-project-button-placeholder
element in order to match the width of the collapse button that has 0.25
+ 0.5 rem of horizontal padding and the svg element inside has 1.25em of
width.

As you can see from the screenshots, the first and third projects are
misaligned compared to the second one. The same happens with
sub-subprojects.

<img width="253" height="129" alt="image"
src="https://github.com/user-attachments/assets/9ca55415-2303-45b5-8a9e-d1b0e03d44bb"
/>
<img width="258" height="258" alt="image"
src="https://github.com/user-attachments/assets/ff6abfbe-5527-48d2-b8aa-2b34da054e19"
/>

Here there are the same projects with this fix applied. The alignment
seems to be preserved even with deeply nested projects.

<img width="247" height="143" alt="image"
src="https://github.com/user-attachments/assets/6566e37a-2017-43d5-b94d-e3daf1a4eb7f"
/>

<img width="279" height="274" alt="image"
src="https://github.com/user-attachments/assets/95d6afd9-ace8-4d55-95c2-c1f924d4826f"
/>

frontend/src/styles/theme/navigation.scss

@@ -85,11 +85,12 @@
         svg {
           transition: all $transition;
           color: var(--grey-400);
+          width: 1.25rem;
         }
       }
 
       .collapse-project-button-placeholder {
-        width: 1.625rem;
+        width: 2rem;
         flex-shrink: 0;
       }