feat(tui): compose and read private messages

Add private messaging to the TUI:
- 'm' opens the command bar in message mode; type '<recipient> <text>' and
  Enter sends it via the client;
- incoming messages are polled each tick into an inbox;
- 'i' toggles a centered inbox popup listing sent/received messages.

The shared command bar now carries a mode (search vs message) so the prefix,
submit action and shortcut hints follow what is being typed.

soulseek-rs/src/main.rs

@@ -172,7 +172,9 @@ fn send_private_message(
 
     client
         .send_private_message(recipient, message)
-        .map_err(|e| color_eyre::eyre::eyre!("Failed to send message: {}", e))?;
+        .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.