[Refactor] Improve the logic of script.js

[castorNova2]

Description

In the UI initialization script responsible for handling beginner/advanced mode, the current implementation uses innerHTML for updating icons and var for variable declarations and used redundant mode check for null.

Expected Behavior

• DOM updates should use textContent instead of innerHTML
• Variables should use const and let instead of var

Current Behavior

UI updates use:

modeIcon.innerHTML = "star";

Uses var for variable declarations

Proposed Changes

Replace innerHTML with textContent:

modeIcon.textContent = "star";

Replace var with const / let

Benefits

Improves performance by avoiding unnecessary HTML parsing
Reduces the risk of HTML injection
Aligns with modern JavaScript best practices

Checklist

• I have read and followed the project's code of conduct.
• I have searched for similar issues before creating this one.
• I have provided all the necessary information to understand and reproduce the issue.
• I am willing to contribute to the resolution of this issue.

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