Add per-tab background image support for Tabs block

[CySSoO]

Motivation

• Allow editors to set a background image for each Tab state so tabs can have independent visual backgrounds and better design control.

Description

• Added a background_image repeater field of type fileupload to the Tabs block configuration in src/Service/EverblockPrettyBlocks.php.
• Updated the Tabs template views/templates/hook/prettyblocks/prettyblock_tab.tpl to render per-tab background images via an inline background-image style with background-size:cover, background-position:center, and background-repeat:no-repeat when present.
• Applied any per-tab css_class into the tab pane element so custom styling can target individual tabs.

Testing

• No automated tests were run for this change.

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Codex Task