Fix: enforce minimum team name length (>=3) in UI

[JSchut96]

Issue
Based on: #3254

The backend enforces a minimum team name length of 3 characters and returns a 400 Bad Request when this requirement is not met (e.g. "Circle name is too short").

However, the UI does not properly handle or display the error message from the server, leaving the user without a clear indication of why their submission failed.

Proposed Solution
Instead of focusing on improving error message display, this fix prevents invalid team names from being submitted altogether.

The solution:

• Ensures the "Create team" button is only enabled when the team name meets the length requirement.
• Prevents the form from being submitted via the Enter key if the name is invalid.
• Communicates the length requirement within the input placeholder and shows a helpful error/hint when the name does not meet the minimum character count.

[DerDreschner]

Hey, thank you very much for your pull request! Could you please change the code so the CI runs are green?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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