Put the wiped entry boxes back on Alt+W

N1MM wipes on Ctrl+W and puts the boxes back on Alt+W, which this had no
key for. A wipe now keeps the call, the exchange, the name, the comment
and the sent time, and Alt+W fills them in again.

Wiping empty boxes keeps what the wipe before it held, and the boxes are
only filled in again when they are empty, so nothing typed since the wipe
is thrown away. A call moved onto the call stack is not kept: it has not
gone anywhere and putting it back would make a second copy of it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015NZkFxHde1iifAQemFTHxf

src/Nonemm.Session/EntryFields.cs

@@ -89,6 +89,12 @@ public sealed class EntryFields
     private static bool IsFilledIn(ExchangeField field) =>
         field.IsSent || field.Kind == ExchangeFieldKind.Report;
 
+    /// True when the operator has typed nothing. The boxes holding what this
+    /// station sends are filled in by the program, so they do not count.
+    public bool IsEmpty =>
+        Call.Trim().Length == 0 &&
+        Exchange.Where((f, at) => !f.IsSent && values[at].Trim().Length > 0).Any() == false;
+
     /// True when everything the contest asks for has been typed.
     public bool IsComplete =>
         Call.Trim().Length > 0 &&