chore(studio): small visual update to empty observability charts (#46728)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Subtle improvement to charts that cannot load data. Less floaty.

| Before | After |
|--------|--------|
| <img width="731" height="323" alt="Screenshot 2026-06-08 at 11 46 21"
src="https://github.com/user-attachments/assets/14c87766-dfe3-448d-8c9c-f3176b658d08"
/> | <img width="791" height="333" alt="Screenshot 2026-06-08 at 11 46
01"
src="https://github.com/user-attachments/assets/d0e33b91-2990-41d2-b14c-25065b1f0c12"
/> |






<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

**Style**
- Improved visual styling for chart error and loading states. Chart
placeholders now feature increased height and enhanced border
presentation with dashed borders and rounded corners, providing clearer
visual distinction when data is unavailable or unable to load.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/studio/components/ui/Charts/ChartHandler.tsx

@@ -149,7 +149,7 @@ const ChartHandler = ({
 
   if (chartData === undefined) {
     return (
-      <div className="flex h-52 w-full flex-col items-center justify-center gap-y-2">
+      <div className="flex h-64 w-full flex-col items-center justify-center gap-y-2 border border-dashed rounded-md">
         <WarningIcon />
         <p className="text-xs text-foreground-lighter">Unable to load data for {label}</p>
       </div>

apps/studio/components/ui/Charts/ComposedChartHandler.tsx

@@ -273,7 +273,7 @@ const ComposedChartHandler = ({
 
   if (!combinedData) {
     return (
-      <div className="flex h-52 w-full flex-col items-center justify-center gap-y-2">
+      <div className="flex h-64 w-full flex-col items-center justify-center gap-y-2 border border-dashed rounded-md">
         <WarningIcon />
         <p className="text-xs text-foreground-lighter">Unable to load data for {label}</p>
       </div>