BE: Messages: Show headers duplicates

[PopClom]

• [ ] Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview) Closes #71 Headers are now always represented as Map<String, List<String>> to support duplicate keys. Single values are wrapped in a list for consistency. Added a tooltip to clarify the new format, as it's not backward-compatible with the previous header structure. Also fixed a typo in the smart filter info modal.

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• [ ] No need to
• [x] Manually (please, describe, if necessary)
• [x] Unit checks
• [ ] Integration checks
• [ ] Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• [x] My changes generate no new warnings (e.g. Sonar is happy)
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged

Check out Contributing and Code of Conduct

A picture of a cute animal (not mandatory but encouraged)

[kapybro[bot]]

Further user feedback is requested. Please reply within 7 days or we might close the issue.

[kapybro[bot]]

Thanks for the additional feedback! We'll get back to your issue soon.

[kapybro[bot]]

Further user feedback is requested. Please reply within 7 days or we might close the issue.

[kapybro[bot]]

No feedback received within 7 days. Auto closing.

[Haarolean]

@PopClom please refrain from force pushing in the future, this marks the whole diff as a new and we can no longer re-review the changed part.