Append to existing className instead of overwriting

[PruthviPraj00]

What Changed:

• Updated logic to handle the class attribute separately.
• Appends the new class value to el.className instead of overwriting it.

Why:

• Prevents loss of existing CSS classes when dynamically setting attributes.
• Ensures that additional classes from data items are added without removing pre-existing ones.

Impact:

• Improves UI consistency and avoids unintended style overrides when multiple class values are applied.

[jahaganiev]

Hey @PruthviPraj00 - thanks for the PR. It'll be included in the next release.

[jahaganiev]

Preline UI v3.2 is live and includes the PR improvements. Thanks!