Visual flaws:

Page load of about 7 Seconds until you see more than a headline. The graphql request takes about 5-6 seconds of that but there is no loading indicator.
No cursor pointer or click indicator on list items
Very minimalist styling
No images in detail view (which were requested in the assignment!)
If you put links in detail view, they should at least be clickable. They are not.
null and undefined printed in the detail view

Implementation flaws:

Too much data in the initial graphql request, all detail in the first request and no pagination (so this is the only request ever sent) - pagination is only in the frontend. (This causes long initial wait)
String concatenations instead of template literals (ES6 basics!)
Mixing of ' and " for strings in code - uniformity!
Not making use of basic react functionality but setting detail view with ...
.innerHTML = ...
Not using npm, no package.json (was not requested, but should be self-evident)
Spaghetti-Code, all in one single file
Not using any library for graphql, but writing the query as string-concatenation
no proper error handling in graphql request
console logs not removed - code style!
No indentations in surrounding HTML frame - code style!
Missing semicolon (line 289) - code style!
No linting / test cases (were not requested but should at least be mentioned as "To be considered before release" - which was requested but is completely missing from the readme)

Magánnyomozót fogunk fogadni, már nem várunk ennyit, mint most.