• Add validation to check if entry type name already exists
• Disable add button when duplicate name is entered
• Implement case-insensitive name comparison

Fixes JabRef#15074

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• I own the copyright of the code submitted and I license it under the MIT license
• [.] I manually tested my changes in running JabRef (always required)
• [.] I added JUnit tests for changes (if applicable)
• [.] I added screenshots in the PR description (if change is visible to the user)
• [.] I added a screenshot in the PR description showing a library with a single entry with me as author and as title the issue number
• [.] I described the change in CHANGELOG.md in a way that can be understood by the average user (if change is visible to the user)
• [.] I checked the user documentation for up to dateness and submitted a pull request to our user documentation repository