Design spec for the case-page half of #14: a single "Kopieren +
Schließen" text button below the document that copies the document
text to the clipboard and then closes the case, landing on /cases.
The H1 trash-can close button is removed; the standalone icon copy
button stays. A failed clipboard write aborts the close so a finished
case never disappears with an empty clipboard.
Frontend-only (one Askama template + its inline script); no Rust
route change — the close handler already strips the case-page Referer
to /cases. The case-list half of #14 is deferred.
refs #14
