Adjust centered Everblock slider behavior and styles

[CySSoO]

Motivation

• Provide a “centered” slider layout where the active slide is visually centered and dominant with attenuated adjacent slides, without adding libraries or changing Prettyblocks config.
• Make navigation move by single slide index so the focused item is always centered after interaction.
• Keep existing slider structure and arrows while ensuring compatibility with PrestaShop 8/9 and no use of .row/.col-*.

Description

• Updated JS in views/js/everblock-slider.js to compute the translateX per the centered formula using containerWidth, itemWidth, gap and index, and to navigate by single-index steps for controls and autoplay (goToIndex and startAutoplay).
• Modified state handling to set state.containerWidth, use state.maxIndex = totalItems - 1, and manage pageCount/pageIndex for per-item centering; added toggles for is-active, is-adjacent, and is-inactive classes in updateItemStates.
• Adjusted button behavior so arrows remain visible (hidden = false) and are only disabled when navigation is not possible, and wired arrow clicks to move one index (-1/+1).
• Added minimal CSS in views/css/everblock.css to include transform in transitions, new .is-adjacent and updated .is-inactive styles (opacity/filter), and moved arrow positions to left: -40px / right: -40px to sit outside the central zone.

Testing

• No automated tests were executed for this change.

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Codex Task