feat: improve field config screen

[slimluccii]

Changes

• Move min/max fields to a separate "Validation" section with type="number" inputs
• Add descriptive hints to all config and mapping fields
• Switch additionalHeaders to a TextareaField for easier JSON editing
• Add missing useCORSProxy to useEffect dependency array
• Debounce setParameters calls (300ms) to reduce unnecessary updates
• Derive preview with useMemo instead of useState + side-effect
• Build parameters object explicitly instead of spreading stale closure

Test

1. Clone repository
2. Run npm ci
3. Run npm run dev
4. Go to DatoCMS -> dato-plugin-tryout -> Schema -> Plugin Foreign Data Selector
5. Edit field. Either fields are fine
6. Go to presentation
7. Validate if config screen is clear.