chore(signer): log pairing start and session failures to stderr

Pairing failures were invisible: fail() wrote nothing and the only
success-path trace was the URI. Backend stderr is captured by Electron,
so a stalled handshake is now diagnosable from /tmp logs.

src/signer/nip46_client.rs

@@ -790,6 +790,8 @@ impl Nip46ClientSigner {
     }
 
     fn fail(&self, message: impl Into<String>) {
+        let message = message.into();
+        eprintln!("[nip46] session failed: {message}");
         if let Ok(mut inner) = self.inner.try_lock() {
             inner.phase = Nip46Phase::Error(message.into());
             inner.task = None;