Reduce z index on row selection header (supabase#46360)

## Context

`RowSelectionHeader` was unnecessarily using a 99 z-index value which
was conflicting with the tooltips, so reducing it down to 2 which should
be all that we need

### Before
<img width="558" height="280" alt="image"
src="https://github.com/user-attachments/assets/362be2df-aca7-4cb9-9bfb-84e77dac06ab"
/>


### After
<img width="374" height="161" alt="image"
src="https://github.com/user-attachments/assets/118c8411-f25c-4513-9cce-2d24dcc71759"
/>


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-->

## Summary by CodeRabbit

* **Style**
* Updated the z-index layering of the selected-rows overlay header
component.

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Author: joshenlim

apps/studio/components/interfaces/UnifiedLogs/RowSelectionHeader.tsx

@@ -61,7 +61,7 @@ export const RowSelectionHeader = () => {
               damping: 30,
               mass: 0.4,
             }}
-            className="pl-4 pr-2 flex items-center justify-between border-t absolute top-0 z-99 bg-surface-75 w-full"
+            className="pl-4 pr-2 flex items-center justify-between border-t absolute top-0 z-2 bg-surface-75 w-full"
             style={{ overflow: 'hidden' }}
           >
             <p className="text-xs">