feat: collapsible grouped fields, blocks, and repeaters

[bobbwal]

Description

Adds collapsible UI for grouped object fields, blocks, and repeaters in admin forms, defaults them to collapsed, and aligns toggle chevrons. New items expand automatically.

Also adds explicit object field layout control via objectLayout ('nested' | 'flat') and clarifies collapsed behaviour in docs (collapsed applies to nested only).

Fixes #634

Changes

• Add group headings for content blocks and collapsible object groups
• Add objectLayout support for object fields:
  • 'nested' (default): grouped/collapsible UI
  • 'flat': expanded, non-collapsible group layout
• Add collapsed option for object group fields (default collapsed)
• Make block and repeater items collapsible with toggle chevrons
• Default items to collapsed; auto‑expand newly added items
• Align chevron rotation classes with toggle logic
• Add E2E coverage for object layout behavior:
  • flat object renders expanded/non-collapsible
  • nested object renders collapsed by default and expands on toggle
• Update docs to clarify objectLayout and collapsed interaction:
  • collapsed is ignored when objectLayout: 'flat'

Testing

• npm run type-check
• npm run e2e
• BASE_URL=http://localhost:8787 npx playwright test tests/e2e/53-object-layout.spec.ts --config tests/playwright.config.ts
• Manual UI verification in admin content forms (blocks + repeaters)

Unit Tests

• Added/updated unit tests
• All unit tests passing

E2E Tests

• Added/updated E2E tests
• All E2E tests passing

Screenshots/Videos

Before:

After:

Checklist

• Code follows project conventions
• Tests added/updated and passing
• Type checking passes
• No console errors or warnings
• Documentation updated (if needed)

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