Show what has gone out in the transmit pane

The transmit pane is now the type-ahead buffer. What has gone to the engine is
in red beside the box and cannot be reached; what is still to go is in the box
and can be typed over, added to or deleted while the engine works through it.
A macro fills the pane rather than the engine, so the operator can correct a
call the message is still sending.

The cursor holds the pump: nothing behind it goes out, so an engine that
catches up with the operator idles instead of transmitting half a word. With
the box out of focus there is no cursor and everything goes.

Escape and the RX button drop what has not gone out and stop the engine where
it is. Enter puts in the carriage return the engine takes as a new line. The
pane starts empty for the next message.

The baud rate the pump counts by is in the digital setup window, beside the
alignment frequency. It has to match the engine: 45.45 unless the contest says
otherwise.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

src/Nonemm.App/Configuration/Settings.cs

@@ -255,6 +255,11 @@ public sealed record Settings
     /// fills the report in, the way typing one does.
     public bool DigitalGrabSendsSpace { get; init; } = true;
 
+    /// The speed the engine transmits at. The type-ahead buffer is paced by
+    /// it, so a station running 75 baud sets it here as well as in the engine.
+    /// MMTTY's own default, and what nearly every RTTY contest runs at.
+    public double DigitalBaud { get; init; } = 45.45;
+
     /// N1MM caps the receive pane's font at 14 points.
     public int DigitalFontSize { get; init; } = 12;