[MOOSE-360] FE: Refactor Horizontal Tabs Block

[GeoffDusome]

What does this do/fix?

Reordering

• Drag-and-drop: Tabs can be reordered by dragging the handle (using @dnd-kit/core and @dnd-kit/sortable with horizontalListSortingStrategy).
• Persistence: Reordering is persisted on save by using moveBlockToPosition so the block tree (and saved content) stays in the correct order.
• Immediate UI update: The tab bar is derived from innerBlocks via useSelect and useMemo instead of syncing into attributes in useEffect, so the list re-renders as soon as the block order changes.

Dynamic block refactor

• PHP render: The block is rendered on the front-end via render.php and no longer uses save.js for the outer markup.
• Save: Only inner blocks are saved (<InnerBlocks.Content /> in the block’s save), so tab order comes from the saved block order.
• Controller: Horizontal_Tabs_Block_Controller in plugins/core/src/Components/Blocks/ builds the tab list from $block->parsed_block['innerBlocks'] so order is a single source of truth and survives save/reload.

Active tab behavior

• Front-end: The first tab is always the active one; the “current active tab” attribute is no longer used in PHP.
• Editor: The editor always opens with the first tab active (using a ref so this runs once per mount and isn’t overwritten when reordering).

Editor UX

• Activate tab when editing label: Focusing the tab label (e.g. clicking to edit) also activates that tab via an onFocus handler that calls onSelectTab(tab.id).

Accessibility

• Tab row: The clickable tab row has role="button", tabIndex={0}, aria-pressed, and an onKeyDown handler so Enter/Space activate the tab; key handling is skipped when focus is inside the label’s contenteditable.

Theming

• Drag handle: The drag handle uses the same Button component and variant="link" as the delete button so it shares theme styles.
• Editor colors: Drag handle and delete button both use --horizontal-tabs-editor-delete-color and --horizontal-tabs-editor-delete-hover-color in editor.pcss so they stay visible in light/dark (and brand) themes.

Files changed

• Theme: horizontal-tabs/edit.js, horizontal-tabs/render.php, horizontal-tabs/editor.pcss, horizontal-tabs/block.json, horizontal-tabs/index.js; removed horizontal-tabs/save.js.
• Plugin: Added Horizontal_Tabs_Block_Controller.php; no changes to the horizontal-tab child block.

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QA

Links to relevant issues

• Link to Issue

Screenshots/video:

• Editor view: https://p.tri.be/i/gqzJIG
• FE view: https://p.tri.be/i/zBpOG8

Pull request checklist

• I've added a changelog entry for these changes.
• I've linked to a relevant Jira issue.
• I've captured a screenshot or screencast of the changes and linked it above.

[dpellenwood]

🟢 This is an impressive improvement to the tabs block, Geoff. Nice work!