Fix: replace deprecated utf8_decode() with mb_strlen() and null coalescing for PHP 8.2

[dewillepl]

This PR replaces the deprecated usage of utf8_decode() with a PHP 8.2+ compatible approach using mb_strlen() and null coalescing operator for better readability and null handling.

Problem:
With the current Dockerfile (FROM php:fpm-alpine), the container installs PHP 8.2 by default. Running the app produces deprecation warnings:

PHP 8.2: utf8_encode() and utf8_decode() functions are deprecated

Change:
Before:

$shortage = 4 - strlen(utf8_decode($input)) % 4;

After:

$shortage = 4 - (mb_strlen($input ?? '', 'UTF-8') % 4);

Benefit:

• Removes deprecation warnings on PHP 8.2+
• Ensures forward compatibility with future PHP versions

[dewillepl]

Previous PR included extra commits by mistake. This one contains only the intended change.