feat: Adding routes handling, always going through ROUTES instead of hardcoded everywhere

[DennisGaida]

Summary

I am fully aware this touches a lot of files, but in the end it all references back to routes.ts. Even if you don't like the URLs I am proposing (/admin/...), these could be easily changed in the routes.ts to the previous URLs/paths, but we would get away from those hardcoded paths.

In this pull request, the following changes are made:

• Implemented routes.ts so all routes go through it for UX/UI and pretty URLs

• Removes all hardcoded URLs / paths everywhere and I hope I haven't missed a spot

• I tried testing all paths and all functionalities as well as setting up a completely new UK instance

• All legacy URLs are being redirected automatically (see routes.js on the bottom) - no broken bookmarks/links

• Documentation & Tests never reference paths directly hence nothing to update

• I don't know how to do CI tests locally, so I didn't do that next to npm run dev, npm run tsc and node server/server.js, maybe there are some automatic GH actions?

• Resolves Consistent routes / URIs for easier security hardening #2505

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
• 🧠 I have disclosed any use of LLMs/AI in this contribution and reviewed all generated content.
  I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

[CommanderStorm]

Looks interesting.
It oes not currenty pass CI though, because of some TS-issue.

[DennisGaida]

• PR title: ❌ can't fix, no permissions to prefix with feat:
• e2e-tests: ✅ fixed, by using new paths hardcoded. Not sure whether I like this, but maybe tests need to have hardcoded paths
• CI tests: ✅ only failed for node 20 through some weird quirk with import("routes") using the ts version instead of the js version. All a bit weird with the *.ts and *.js files lying in the same folder and manually having to adjust these files as stated in the util.js comment.
• linters: ✅ fixed (would be fixed easily if tsconfig.json was just configured with "removeComments": false because tsc is actually throwing the comments away which the linters want again - I changed this setting now for tsconfig-backend so the linters don't complain when throwing JSDoc comments away)