Clarify username taken error message during registration

[belugaaaaaaaa]

Summary
This PR clarifies the error message displayed during user registration when a selected username conflicts with an existing user or group name.
Previously, the client-side validation showed a generic “username taken” message, which did not clearly indicate that group name conflicts are also checked during registration. This could lead to user confusion when a username was unavailable for reasons other than an existing user account.
Changes
Updated public/src/client/register.js to use a more precise translation key when a username or group name conflict is detected during registration.
Added a corresponding error message entry in public/language/en-US/error.json to clearly communicate the cause of the conflict to the user.
Imapact
This change improves the accuracy of user-facing feedback during registration without altering backend logic. By making the error message more explicit, it reduces ambiguity and helps users better understand why a chosen username cannot be used.

[coveralls]

Pull Request Test Coverage Report for Build 21612115417

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.841%

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Totals
Change from base Build 21503398961:	0.0%
Covered Lines:	25358
Relevant Lines:	30328

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💛 - Coveralls