fix(svelte): Center file tree loading indicator (#64309)

We should probably redesign the whole loading state but this simple
change is still better than the current behavior.

| Before | After |
|--------|--------|
|
![2024-08-06_21-00_1](https://github.com/user-attachments/assets/015d47d3-9542-4d72-b422-56975723576c)
|
![2024-08-06_21-00](https://github.com/user-attachments/assets/0e667bc2-496e-4169-806d-b49757681673)
|

## Test plan

Manual testing.

Author: fkling

client/web-sveltekit/src/routes/[...repo=reporev]/(validrev)/(code)/+layout.svelte

@@ -248,7 +248,7 @@
                         />
                     {/if}
                 {:else}
-                    <LoadingSpinner center={false} />
+                    <LoadingSpinner />
                 {/if}
             </div>
         </div>