feat: Ensure Unique Usernames During Profile Updates

[othbcq]

This PR implements the functionality to enforce unique usernames when users update their profile information. Previously, while registration ensured unique usernames, profile updates did not. This led to potential duplication and conflicts. The changes address this issue comprehensively.

Changes Included:

• Username Uniqueness Check:

  • Added backend logic to verify that a new username is not already taken before allowing updates.
  • Integrated this check for both authentication methods (Email/Password and Google Sign-In).

• Real-Time Feedback:

  • Enhanced the frontend to provide immediate feedback if the desired username is unavailable.

• Consistent Enforcement:

  • Ensured that the uniqueness constraint is consistently applied across all authentication flows.

Acceptance Criteria Met:

• Users cannot update their profile with a username that already exists.
• Users receive clear and immediate feedback when attempting to use an unavailable username.
• The uniqueness check is enforced consistently for all users.

[sonarqubecloud]

Quality Gate failed

Failed conditions
48.48% Line Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[othbcq]

Due to insufficient code coverage and a lack of time to write additional tests, we have decided to close this PR.