Fix simple image slider arrow positioning

[CySSoO]

Motivation

• Arrow controls in the Prettyblock Simple Image slider could be misaligned when the active slide image size differs from the computed item width, so the design needs the active width to reflect the actual rendered image.

Description

• Added a new updateActiveWidth function in views/js/everblock-slider.js that computes the active width from the currently active slide's rendered img width with fallbacks to the item width and previous multiplier.
• Replaced the previous fixed --ever-slider-active-width assignment in updateState with a call to the new updateActiveWidth so arrow positions are recalculated whenever the slider state updates.
• Collected slide images during setupSlider and attached load event listeners to each image to call updateState after images finish loading so the active width is refreshed once images are rendered.

Testing

• No automated tests were executed for this change.

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Codex Task