This commit adds utility functions to support partial settings save functionality:
- isObject: Type guard for plain objects
- deepMerge: Recursively merges objects, preserving unspecified fields
- validateEditableFields: Validates settings against UI editable field whitelist
- UI_EDITABLE_FIELDS: Whitelist of fields modifiable through UI
A failing test is included to verify the deep merge behavior.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
