refactor: Make Fingerprint not implement Display (#8177)

Currently, the Fingerprint type implements Display, but this doesn't get
you the canonical fingerprint representation, but something
human-readable. This is confusing, and back when I first used
`Fingerprint`, I immediately wrote a bug because of this. So, instead,
make a function `human_readable()` on Fingerprint.

This comes from the discussion at
https://github.com/chatmail/core/pull/8174#discussion_r3143130722.

src/qr.rs

@@ -645,7 +645,7 @@ async fn decode_openpgp(context: &Context, qr: &str) -> Result<Qr> {
         }
     } else {
         Ok(Qr::FprWithoutAddr {
-            fingerprint: fingerprint.to_string(),
+            fingerprint: fingerprint.human_readable(),
         })
     }
 }