feat(registry): optimize default nickname logic for user participants

[bytedaring]

Updated the fallback key for user participant default names from common.you to profile.defaultNickname. This ensures consistency with the default values used on the profile page.

Summary

This PR refactors the default nickname acquisition logic for user participants. By switching the fallback key from common.you to profile.defaultNickname, we ensure that the user's display name remains consistent across the Registry and the Profile modules.

Related Issues

N/A

Changes

• Modified the naming fallback logic in the participant registry.
• Updated the i18n/config key reference from common.you to profile.defaultNickname.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or build changes

Verification

Steps to reproduce / test

1. Log in to the application and navigate to the participant list (Registry).
2. Find a user participant who has not set a custom nickname.
3. Observe the fallback name displayed and verify it matches the value in profile.defaultNickname (instead of common.you).

What you personally verified

• Verified that the fallback logic correctly triggers when the primary nickname is missing.
• Manually confirmed that the change aligns with the Profile page's display logic.

Evidence

• CI passes (pnpm check && pnpm lint && npx tsc --noEmit)
• Manually tested locally

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added/updated documentation as needed
• My changes do not introduce new warnings