feat: gate first contact on a pinned server, confirm key changes

src/demo.rs

@@ -39,6 +39,11 @@ fn plant(dir: &Path) -> Result<(), String> {
     let me = Account::new(master, 0).map_err(|e| e.to_string())?;
     let addr = Address::parse("demo@carrier.local").map_err(|e| e.to_string())?;
     store.set_account(&addr).map_err(|e| e.to_string())?;
+    // Our own server is pinned, the contacts' server is not: the two
+    // states the trust lines and the pin prompts distinguish.
+    store
+        .pin_server(&addr.host, addr.port, &identity(42).pk())
+        .map_err(|e| e.to_string())?;
 
     let pk = me.me().pk();
     let mut read_ids: Vec<String> = Vec::new();