fix(ui): sidebar missing sticky top offset (#13652)

Regression from https://github.com/payloadcms/payload/pull/13340.
Incorrect fallback value for the `--sidebar-wrap-top` css variable was
used.

### Before
<img width="2896" height="1200" alt="CleanShot 2025-09-01 at 10 03
39@2x"
src="https://github.com/user-attachments/assets/caf054d2-ac61-4a96-b6a1-3981ce9f528f"
/>

### After
<img width="2894" height="1194" alt="CleanShot 2025-09-01 at 10 02
59@2x"
src="https://github.com/user-attachments/assets/1e13e084-6f62-47bb-88b3-921ba5f3ff10"
/>

packages/ui/src/elements/DocumentFields/index.scss

@@ -93,7 +93,7 @@
       }
 
       position: var(--sidebar-wrap-position, sticky);
-      top: var(--sidebar-wrap-top, 0);
+      top: var(--sidebar-wrap-top, var(--doc-controls-height));
       width: var(--sidebar-wrap-width, 33.33%);
       height: var(--sidebar-wrap-height, calc(100vh - var(--doc-controls-height)));
       min-width: var(--sidebar-wrap-min-width, var(--doc-sidebar-width));