Check the entry boxes by the rules the log window uses

The entry window took any text and only the log window checked it, so the
same value was accepted in one place and refused in the other. The rules
now live in one file, ExchangeCheck, which both windows call.

OperatingPosition.EntryProblem names the first box that cannot be logged
and why. LogContact and SaveQuickEdit throw on one, so a caller that has
not checked cannot write nonsense to the log. The entry window checks
before it logs, puts the reason in the status line and the cursor in the
box to fix, which covers Enter, the Log It button, ESM and {LOG}.

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

src/Nonemm.App/Windows/EntryWindow.axaml.cs

@@ -490,7 +490,7 @@ public sealed partial class EntryWindow : Window
     /// both do.
     private void SaveQuickEdit()
     {
-        if (Logging?.SaveQuickEdit() is not { } saved)
+        if (!IsEntryUsable() || Logging?.SaveQuickEdit() is not { } saved)
         {
             return;
         }
@@ -502,7 +502,7 @@ public sealed partial class EntryWindow : Window
 
     private void LogContact()
     {
-        if (Logging is null)
+        if (Logging is null || !IsEntryUsable())
         {
             return;
         }
@@ -521,6 +521,22 @@ public sealed partial class EntryWindow : Window
         boxes[0].Focus();
     }
 
+    /// True when everything typed can go in the log. A value the contest will
+    /// not take puts the reason in the status line and the cursor in its box,
+    /// which is where the operator has to fix it.
+    private bool IsEntryUsable()
+    {
+        if (Logging?.EntryProblem() is not { } problem)
+        {
+            return true;
+        }
+        Status(problem.Message);
+        Logging.Entry.FocusOn(problem.Field);
+        boxes[problem.Field].Focus();
+        boxes[problem.Field].SelectAll();
+        return false;
+    }
+
     /// What space does: the callsign box and the exchange boxes, stepping over
     /// the reports and over a box this station is not asked for, and round to
     /// the callsign again. This is N1MM's NextTab.