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/contact.rs

@@ -1396,7 +1396,7 @@ WHERE addr=?
         let Some(fingerprint_other) = contact.fingerprint() else {
             return Ok(stock_str::encr_none(context));
         };
-        let fingerprint_other = fingerprint_other.to_string();
+        let fingerprint_other = fingerprint_other.human_readable();
 
         let stock_message = if contact.public_key(context).await?.is_some() {
             stock_str::messages_are_e2ee(context)
@@ -1410,7 +1410,7 @@ WHERE addr=?
         let fingerprint_self = load_self_public_key(context)
             .await?
             .dc_fingerprint()
-            .to_string();
+            .human_readable();
         if addr < contact.addr {
             cat_fingerprint(
                 &mut ret,