Introduce checkstyle (or similar) plugin and make it part of CI pipeline

I think the majority of small code review comments could be avoided if we had strict style checking as part of the build. It would also keep the code consistent. I suggest we introduce checkstyle, take an initial hit on having to reformat/update some code and live happily ever after.

Things checkstyle should enforce (lets update this list as cleanup continues):
- [ ] redundant access modifiers in interfaces
- [ ] four-space indentation
- [ ] diamond for templates on the right hand side of assignments
- [ ] final modifier
- [ ] line length

Start by inspecting the project's build and CI configuration to find where a Checkstyle or similar plugin would be integrated. Review the requested rules—interface access modifiers, four-space indentation, diamond operators, final modifiers, and line length—and determine the existing cleanup scope. Done means the checks run in CI and the initial violations are resolved.

Written by the indexing model from the issue text.