Enhancement: Collapsible UI for block and repeater items

[bobbwal]

Summary

Add collapsible headers to block and repeater items in admin content forms, defaulting to collapsed to reduce vertical
clutter.

Motivation

• Forms with many blocks/repeater items become hard to scan.
• Collapsing by default makes long content more manageable.
• Users can expand only the item they’re working on.

Proposed behaviour

• Block items and repeater items render collapsed by default.
• Title area + toggle button expand/collapse the item.
• Newly added items expand automatically so users can fill them out.
• Chevron state reflects expanded/collapsed state.

Affected areas

• Admin content form: blocks field + structured array (repeater) field

Testing

• Manual: add block/repeater items, confirm default collapsed and auto‑expand on add.