build(deps): Upgrade all dependencies and eliminate unsound chain

Bump clap to 4.6.6, imap to 3.0.0-alpha.15 and native-tls to 0.2.18.
The imap upgrade removes the unsound lexical-core 0.7.6 dependency
inherited via nom 5 (RUSTSEC-2023-0086), leaving cargo audit clean.
Drop the unused imap-proto dependency and migrate to the imap 3.x
ClientBuilder and AppendCmd APIs.

src/imap_client/mod.rs

@@ -1,8 +1,7 @@
 use crate::config::settings::ImapConfig;
-use native_tls::TlsConnector;
 
 pub struct ImapClient {
-    session: Option<imap::Session<native_tls::TlsStream<std::net::TcpStream>>>,
+    session: Option<imap::Session<imap::Connection>>,
 }
 
 impl ImapClient {
@@ -13,8 +12,7 @@ impl ImapClient {
     }
 
     pub fn login(&mut self, config: &ImapConfig) -> Result<(), Box<dyn std::error::Error>> {
-        let tls = TlsConnector::builder().build()?;
-        let client = imap::connect((config.host.as_str(), config.port), &config.host, &tls)?;
+        let client = imap::ClientBuilder::new(config.host.as_str(), config.port).connect()?;
         let session = client.login(&config.username, &config.password).map_err(|e| e.0)?;
         self.session = Some(session);
         Ok(())
@@ -29,7 +27,7 @@ impl ImapClient {
 
     pub fn append_message(&mut self, message: &str) -> Result<(), Box<dyn std::error::Error>> {
         if let Some(ref mut session) = self.session {
-            session.append("INBOX", message.as_bytes())?;
+            session.append("INBOX", message.as_bytes()).finish()?;
         }
         Ok(())
     }