fix: initialize slider when DOM is already ready

[leno23]

Summary

• replace the slider block's nested DOMContentLoaded listener with a ready-state aware initializer
• keep the Swiper variable scoped inside an IIFE
• allow the generated frontend script to initialize whether it is wrapped by the post-assets loader or executed after DOM readiness

Test plan

• php -l /tmp/wp-spectra-slider-frontend.patched.php
• git diff --no-index --check /tmp/wp-spectra-slider-frontend.orig.php /tmp/wp-spectra-slider-frontend.patched.php

Fixes #52