Add vanilla everblock slider frontend and allow script

[CySSoO]

Motivation

• Restore and implement a frontend slider mode for the Simple Image prettyblock using vanilla JS and Bootstrap utilities so it works without external dependencies.
• Provide a responsive, accessible and configurable slider supporting per-breakpoint items, autoplay and infinite looping.
• Allow the new slider script to be distributed/loaded by adding it to the module allowed files list.

Description

• Add a new vanilla slider implementation in views/js/everblock-slider.js that reads data-* attributes (data-items, data-items-tablet, data-items-mobile, data-autoplay, data-autoplay-delay, data-infinite), computes item widths, handles prev/next, dots pagination, autoplay, resize and uses transform: translateX() for movement.
• Register the slider script for frontend inclusion by calling registerJavascript for modules/<module>/views/js/everblock-slider.js in everblock.php.
• Add minimal styling for the slider track, controls and dots in views/css/everblock.css (classes like .ever-slider-track, .ever-slider-button, .ever-slider-dot).
• Update views/templates/hook/prettyblocks/prettyblock_img.tpl to render the slider markup (.ever-slider, .ever-slider-track, .ever-slider-item, prev/next buttons and .ever-slider-dots) and wire the data attributes and gap classes for slider mode.
• Add views/js/everblock-slider.js to config/allowed_files.php so the file is whitelisted for distribution.

Testing

• No automated tests were executed for these changes (no CI or test suite run).
• Local manual verification steps are not included in this PR.

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Codex Task