fix(mermaid): use absolute positioning for action buttons to fix clicks in nested scroll containers

Same fix as code-block (ab0a5ae): switches the mermaid block action button
wrapper from position:sticky to position:absolute so that hit-testing works
correctly in layouts with multiple nested overflow:auto scroll containers.

The mermaid block container already has position:relative, so only the
inner toolbar wrapper needed updating.

Fixes vercel#494

Author: sleitor

packages/streamdown/lib/components.tsx

@@ -894,7 +894,7 @@ const CodeComponent = ({
           {shouldShowMermaidControls ? (
             <div
               className={cn(
-                "pointer-events-none sticky top-2 z-10 -mt-10 flex h-8 items-center justify-end"
+                "pointer-events-none absolute top-2 right-2 z-10 flex items-center"
               )}
             >
               <div