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).

This PR also adds session-level collapsed/expanded state persistence for grouped fields, repeaters, and blocks (scoped per document/collection), plus validation visibility improvements so hidden errors are revealed.

Fixes #634

Includes fixes from #649 and #645 (merged into this branch).

Changes

• Add group headings for content blocks and collapsible object groups
• Add objectLayout support for object fields (nested default, flat non-collapsible)
• 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 and duplicated items
• Persist collapsed/expanded state for objects, repeaters, and blocks within session
• Scope persisted UI state by collection/document to avoid cross-collection leakage
• Keep persisted state stable through reorder flows
• Improve validation visibility for collapsed sections (reveal invalid path and focus first invalid control)
• Align chevron rotation classes with toggle logic
• Update docs for objectLayout + collapsed behavior
• Add/update E2E coverage:
• tests/e2e/53-object-layout.spec.ts
• tests/e2e/55-collapsible-state-persistence.spec.ts
• tests/e2e/56-collapsible-validation-visibility.spec.ts
• Update hardening in related E2E flows (54-hero-cta-style-persistence.spec.ts)

Testing

• npm run type-check
• tests/e2e/53-object-layout.spec.ts
• tests/e2e/55-collapsible-state-persistence.spec.ts
• tests/e2e/56-collapsible-validation-visibility.spec.ts
• tests/e2e/54-hero-cta-style-persistence.spec.ts
• Manual UI verification in admin content forms (objects + repeaters + blocks)

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)

Follow-up planned: refactor collapsible-state persistence helpers to reduce duplication and simplify key/state handling (no functional change).

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[bobbwal]

Need to investigate further but it seems that in the grouped/blocks form setup, validation errors inside collapsed groups can be hidden, so Update / Update & Publish can appear unresponsive. This is a UX issue rather than a save failure. Follow-up could auto-expand groups with errors, show an error badge/count on collapsed headers, and focus/scroll to the first invalid field on submit.

[bobbwal]

Need to investigate further but it seems that in the grouped/blocks form setup, validation errors inside collapsed groups can be hidden, so Update / Update & Publish can appear unresponsive. This is a UX issue rather than a save failure. Follow-up could auto-expand groups with errors, show an error badge/count on collapsed headers, and focus/scroll to the first invalid field on submit.

This has been sorted, along with the following:

Update since original PR description:

• Added per-document/collection session persistence for collapsed state across object groups, repeaters, and blocks
• Added collapsed validation reveal behavior for hidden invalid fields
• Added E2E coverage for state persistence and validation visibility
• Addressed follow-up review issues around key scoping and nested object serialization host resolution