feat(cli): add 'message' subcommand to send private messages

`soulseek-rs message <username> <text>` connects, logs in, sends a private
message and exits. Smoke-tested against a local soulfind server.

soulseek-rs/src/cli.rs

@@ -89,6 +89,15 @@ pub enum Commands {
         )]
         max_concurrent_downloads: usize,
     },
+
+    /// Send a private message to another user
+    Message {
+        /// Username of the recipient
+        username: String,
+
+        /// Message text to send
+        message: String,
+    },
 }
 
 pub fn parse_server_address(server: &str) -> color_eyre::Result<(String, u16)> {

soulseek-rs/src/main.rs

@@ -89,6 +89,21 @@ fn main() -> Result<()> {
             };
             search_and_download(config)
         }
+        Some(Commands::Message {
+            username: recipient,
+            message,
+        }) => send_private_message(
+            &ClientSettings {
+                username,
+                password,
+                server_address: PeerAddress::new(server_host, server_port),
+                enable_listen: !cli.disable_listener,
+                listen_port: cli.listener_port,
+                shared_directory,
+            },
+            &recipient,
+            &message,
+        ),
         None => {
             use ratatui::crossterm::{
                 event::EnableMouseCapture,
@@ -139,6 +154,34 @@ fn main() -> Result<()> {
     }
 }
 
+fn send_private_message(
+    settings: &ClientSettings,
+    recipient: &str,
+    message: &str,
+) -> Result<()> {
+    let mut client = Client::with_settings(settings.clone());
+    client
+        .connect()
+        .map_err(|e| color_eyre::eyre::eyre!("Failed to connect: {}", e))?;
+    if !client
+        .login()
+        .map_err(|e| color_eyre::eyre::eyre!("Failed to login: {}", e))?
+    {
+        return Err(color_eyre::eyre::eyre!("Login rejected by server"));
+    }
+
+    client
+        .send_private_message(recipient, message)
+        .map_err(|e| color_eyre::eyre::eyre!("Failed to send message: {}", e))?;
+
+    // The send is dispatched asynchronously; give the server actor a moment to
+    // flush it to the socket before we drop the client and exit.
+    std::thread::sleep(Duration::from_millis(500));
+
+    println!("✉️  Message sent to {recipient}");
+    Ok(())
+}
+
 fn search_and_download(config: SearchConfig) -> Result<()> {
     if config.verbose > 0 {
         println!(