[1.0] Support complex publish inputs with a JSON fallback

The publish form passes unsupported schema shapes as text and the API client loses validation detail. Nested Pydantic inputs are consequently difficult to submit or diagnose.

Scope

• Keep supported simple controls and provide a JSON fallback for complex or unsupported schema forms.
• Preserve schema defaults and parse structured values before publication.
• Surface relevant FastAPI/Pydantic validation locations and messages while keeping protected details private.

Acceptance criteria

• A representative nested model with arrays/objects and defaults publishes with correctly typed values.
• Invalid JSON and schema validation failures give actionable field/path feedback without claiming publication succeeded.
• The fallback is keyboard usable and simple forms continue to work.

Boundaries

No general schema-form framework or new form dependency.

References

• Parent topic: #374.
• src/flock/frontend/src/components/controls/PublishControl.tsx
• src/flock/frontend/src/services/api.ts

Start by reading src/flock/frontend/src/components/controls/PublishControl.tsx and src/flock/frontend/src/services/api.ts to trace publish values and validation responses. Done means nested arrays/objects with defaults publish with correct types, invalid input reports actionable paths without false success, keyboard use works, and simple controls remain unchanged.