Suggest alternative username during registration when username is taken

[RogerJinIS]

Summary
When a user selects a username that is already taken during registration, NodeBB currently only displays a generic "Username taken" error. This pull request improves the registration experience by suggesting an alternative username derived from the user’s original input.

Changes

• Updated the "username-taken" error message to support a suggested alternative
• Modified the registration client logic to pass a suggested username into the error message
• Displays a more helpful error message such as: "Username taken, maybe try test123suffix_123"