Edit and delete contacts in the log

Double-click a cell in the log window to change it. The columns follow the
contest exchange instead of being fixed in the XAML, so CQ WW gets a Zone
column and Sweepstakes gets Nr, Prec, Ck and Sec.

QsoEditor holds the column list and applies one field edit. Validation comes
from the ExchangeFieldKind the contest declared: a CQ zone is 1 to 40, an ITU
zone 1 to 90, a section is looked up in the ARRL list, a grid must parse.
A refused edit returns the reason and leaves the log alone, so the cell
reverts. Changing the callsign runs the country lookup again.

Delete, or the right-click menu, removes the selected contact after a
confirmation. Either way the log is rescored, so a multiplier the removed
contact held passes to the next contact that claims it.

Both go out to the other stations in N1MM's own messages: contactreplace
carries oldcall and oldtimestamp, contactdelete names the contact. An incoming
edit or delete is matched by contact id first, falling back to call plus
timestamp because N1MM does not know our ids.

Country, continent and the two prefixes were filled in twice, once when
logging and once when editing. They now come from CountryFields.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/Nonemm.Session/QsoChange.cs

@@ -0,0 +1,7 @@
+using Nonemm.Core;
+
+namespace Nonemm.Session;
+
+/// An edited contact, plus the call and time it had before the edit. Other
+/// stations look their copy up by that pair.
+public sealed record QsoChange(Qso Qso, string OldCall, DateTime OldTimestampUtc);