[TOOL-3430] SDK: Fix ConnectEmbed showing a flash of Sign In prompt screen even if user is already signed in (#6262)

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## PR-Codex overview
This PR focuses on improving the user experience in the `ConnectEmbed` component by addressing the display of a "Sign In" prompt when the login status is uncertain. Instead, it shows a loading indicator during this period.

### Detailed summary
- Updated the condition for rendering the loading screen in `SignatureScreen.tsx`.
- The loading screen is now shown if either `wallet` is not available or `siweAuth.isLoading` is true, enhancing user feedback during the login process.

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Author: MananTank

.changeset/warm-eggs-relate.md

@@ -0,0 +1,5 @@
+---
+"thirdweb": patch
+---
+
+Fix ConnectEmbed briefly showing "Sign In" prompt while the login status is not known - show loading indicator instead

packages/thirdweb/src/react/web/ui/ConnectWallet/screens/SignatureScreen.tsx

@@ -69,7 +69,7 @@ export const SignatureScreen: React.FC<{
     }
   }, [onDone, siweAuth]);
 
-  if (!wallet) {
+  if (!wallet || siweAuth.isLoading) {
     return <LoadingScreen data-testid="loading-screen" />;
   }