Implement VisaViewController and Thymeleaf templates for Case Management

[eeebbaandersson]

Now that the VisaService and Comment logic are implemented, we need a web interface to allow users and admins to interact with visa cases.

This issue covers the creation of the VisaViewController and the initial Thymeleaf templates for listing/viewing visa details.
It also includes the forms to apply for a new visa and the logic for admins to update/handle existing applications (e.g., assigning a handler or changing status).

[coderabbitai]

🔗 Related PRs

#1 - Infrastructure/update base [merged]
#16 - Feature/comment logic [closed]

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