Empty "Style Instruction" triggers 422 error / [object Object]
Bug Report: Empty "Style Instruction" triggers 422 error / [object Object] Leaving the "Style Instruction (Optional)" input empty causes POST /api/v1/custom-voice/generate to fail with 422 Unprocessable Entity (displayed as [object Object] in the UI). Entering any text or selecting a preset chip (like "Happy") resolves it. It seems the backend Pydantic model expects a non-empty string or is missing Optional[str] = None / empty string handling for that field.
