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/models/mod.rs

@@ -2,6 +2,7 @@ mod app_state;
 mod file_display_data;
 
 pub use app_state::{
-    AppState, DownloadEntry, FocusedPane, SearchEntry, SearchStatus,
+    AppState, ChatMessage, CommandBarMode, DownloadEntry, FocusedPane,
+    MessageDirection, SearchEntry, SearchStatus,
 };
 pub use file_display_data::FileDisplayData;