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.

README.md

@@ -38,7 +38,7 @@ experience. For me, this is a good balance between learning and practicality.
 - [ ] Share files
 - [ ] Browse user(s) files
 - [ ] Chat in chatrooms
-- [ ] Private messaging
+- [x] Private messaging
 - [ ] Headless mode daemon mode with remote control
 
 ## Project Structure