fix: encapsulate modal-related code in an iife to prevent variable leakage

[sentry-autofix]

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This fix was triggered by DB

Fixes BLT-JAVASCRIPT-40-BT

This change addresses a potential issue where variables defined within the modal-related code block could leak into the global scope, potentially causing conflicts with other parts of the application.

• Encapsulated the modal-related code within an Immediately Invoked Function Expression (IIFE) to create a private scope for the variables.
• Added a check for the existence of the welcome message element before attempting to modify its display property.

If you have any questions or feedback for the Sentry team about this fix, please email [email protected] with the Run ID: 9004.