feat(rda): measure the two district sources before arbitrating between them

A log imported from HAMLOG.online carries the Russian district in CNTY, and the
offline RDA database has its own answer for the same contact. Which wins is a
real question — but only if the two are independent. If HAMLOG's value is itself
a lookup in the same kind of database, there is nothing to arbitrate and a
precedence rule would be ceremony around a redundancy.

So this measures instead of deciding: Settings → RDA gains a read-only compare
that reports agreements, disagreements, and the contacts where only one side
knows, then lists the disagreements — callsign, date, both districts, whether
the database answer was a DATED record (the award administrators' own fact) or
the callsign's current district (an assumption), and whether the QSO is
confirmed on HAMLOG (their value has then been through a match of two logs).

It changes nothing in the log. What to do about a conflict is the next decision,
and it should be taken with the operator's own numbers in hand.

Also: the four HAMLOG fields become filterable, so 'never sent there' is a
filter rather than a guess. A field that can be written and not read back is
half a feature — pinned by a test against the bulk-editable list.

frontend/wailsjs/go/main/App.d.ts

@@ -127,6 +127,8 @@ export function ClusterSpotStatuses(arg1:Array<main.SpotQuery>):Promise<Array<ma
 
 export function CompactDatabase(arg1:string):Promise<main.CompactResult>;
 
+export function CompareRDASources():Promise<main.RDACompareResult>;
+
 export function ComputeQSOAwardRefs(arg1:qso.QSO):Promise<Array<main.QSOAwardRef>>;
 
 export function ComputeStationInfo(arg1:string,arg2:string):Promise<main.StationInfoComputed>;