Say when the entry has two signals on the air

N1MM blocks the transmitter when a multi-operator entry is about to break
its own class. This program does not touch the transmitter: a logger that
decides for itself when the operator may not send loses a contact every
time it is wrong, and it is wrong whenever it has stale information about
the other stations — which, on a network, is whenever a laptop has just
been unplugged.

So the entry window says so instead. While this station is transmitting
and the entry is over what it is allowed, a red box goes across the top of
the window naming the station and the band. It is above the frequency and
under the menu, so nothing it covers is anything the operator needs while
he is sending, and it is big enough to read from the other side of the
room.

`TransmitRules` is what it asks:

| Entry | Allowed |
|---|---|
| one transmitter | one signal |
| two transmitters | two signals, on different bands |
| anything else | one signal per band |

Two on one band is wrong in every multi-operator class, so it is checked
whatever the category says. A station that has not said which band it is on
counts as somewhere else: a guess there puts the box in front of an
operator doing nothing wrong.

This is also the first thing that sends `XMIT`. The message was written and
nothing called it, so the transmit column of everybody's network status
window stayed empty and no station knew when another was sending. The entry
window now says when its key goes down and what it is sending.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PdAYHcdRktqKry7nk414TU

src/Nonemm.App/Windows/EntryWindow.axaml

@@ -127,6 +127,15 @@
       </MenuItem>
     </Menu>
 
+    <!-- Two signals where the entry may have one. N1MM blocks the
+         transmitter; this says so instead, loudly enough to be read from
+         across the room, and leaves the operator to stop -->
+    <Border DockPanel.Dock="Top" Name="TransmitWarning" IsVisible="False"
+            Background="#C0392B" Padding="10,8">
+      <TextBlock Name="TransmitWarningText" FontSize="22" FontWeight="Bold"
+                 Foreground="White" TextWrapping="Wrap" Text="" />
+    </Border>
+
     <!-- N1MM's status strip: what just happened on the left, then the band
          change timer, the contacts and multipliers, and the score -->
     <Border DockPanel.Dock="Bottom" Padding="6,3" BorderThickness="0,1,0,0"