Use b for building and r for recruitment

- Replace Ctrl+Space with `b` and Alt+Space with `r` in README and
  documentation.
- Remove the old Alt+Space recruitment keybinding and adjust key
  handling code.
- Revamp skirmish view to display colored lines and updated status
  messages.
- Increase test suite to 83 tests and update test expectations for new
  shortcuts.
- Update flake.lock with the latest revision and hash.

tests/app.rs

@@ -82,8 +82,8 @@ fn key_release_does_not_trigger_quit() {
 fn music_keys_send_commands_without_opening_audio_device() {
     let (mut app, rx) = app();
     press(&mut app, KeyCode::Char('m'));
-    press(&mut app, KeyCode::Char('b'));
-    press(&mut app, KeyCode::Char('n'));
+    press(&mut app, KeyCode::Char('+'));
+    press(&mut app, KeyCode::Char('-'));
     assert!(matches!(rx.try_recv().unwrap(), AudioCmd::Mute));
     assert!(matches!(rx.try_recv().unwrap(), AudioCmd::VolumeUp));
     assert!(matches!(rx.try_recv().unwrap(), AudioCmd::VolumeDown));