fix(messages): acknowledge stored private messages, and disconnect cleanly

Two bugs an e2e test found the moment one existed for this path.

A private message the server held while we were offline arrives with
new_message false, and the handler only acknowledged the ones flagged
new — so exactly the messages that need an ack never got one and the
server re-delivered them at every login, forever. Every delivery is now
acknowledged.

A dropped Client also left its server connection open, so the session
lived on: mail addressed to us went to a socket nobody was reading and a
later login collided with our own ghost. Client::disconnect closes the
server and peer connections, and Drop calls it.

Also covers two features that were never driven end to end: an offline
message delivered once and not again, and a wishlist search answered like
any other.

soulseek-rs-lib/src/client/mod.rs

@@ -1163,6 +1163,12 @@ pub struct Client {
     session: SessionWatch,
 }
 
+impl Drop for Client {
+    fn drop(&mut self) {
+        self.disconnect();
+    }
+}
+
 impl Client {
     pub fn new(
         username: impl Into<String>,