put: report through the view and ask the host-key question through the Prompter

put() loses its MultiProgress parameter: the summary, pre-flight and
connection spinners become view messages, the per-file SFTP transfer
reports determinate progress through view.progress (upload_file takes
a byte-count callback instead of an indicatif bar), and the display is
released through view.suspend on every exit path. The hardcoded
trust-on-first-use prompt in the SSH host-key verification becomes the
Prompter::accept_host_key port (fail-closed by default; the terminal
prompter prints the authenticity banner and confirms), so a remote
frontend can surface its own host-key dialog.

src/report.rs

@@ -147,6 +147,15 @@ pub trait Prompter: Send + Sync {
     ) -> Result<String, Box<dyn Error>> {
         Ok(default.to_string())
     }
+
+    /// Ask whether to accept and store an unverified SSH host key (trust on
+    /// first use): `host` is the display form (`host` or `[host]:port`),
+    /// `key_type` the key type name ("ssh-ed25519", ...) and `fingerprint`
+    /// the human-readable digest. Fail-closed: implementations that cannot
+    /// ask anyone answer `false`, refusing the connection.
+    fn accept_host_key(&self, _host: &str, _key_type: &str, _fingerprint: &str) -> bool {
+        false
+    }
 }
 
 /// Inert view and prompter: drops every event and answers every question