Fix user registration initialization in Go API (#15349)

### What problem does this PR solve?

This PR fixes several behavior gaps in the Go implementation of the user
registration API.

### Type of change

- Make `nickname` required for user registration.
- Align registration error messages and response data with expected API
behavior.
- Handle password decryption errors for registration more consistently.
- Generate UUID v1-style IDs for new users, access tokens, tenants,
user-tenant records, and root files.
- Initialize default user fields during registration, including:
  - language
  - color schema
  - timezone
  - last login time
- Create user, tenant, user-tenant relation, tenant LLM records, and
root folder in a single DB transaction.
- Initialize default tenant LLM records from configured default models.
- Avoid partial registration data when one creation step fails.
- Use locale-based default language fallback for user profile responses.