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/src/components/SettingsModal.tsx

@@ -13,7 +13,7 @@ import {
   ListProfiles, GetActiveProfile, SaveProfile, DeleteProfile, ActivateProfile, DuplicateProfile,
   GetRotators, SaveRotators, TestRotatorDevice, RotatorPark, RotatorStop,
   GetRotorPresets, SaveRotorPresets, ResetRotorPresets,
-  GetUltrabeamSettings, SaveUltrabeamSettings, TestUltrabeam, CompactDatabase, CheckHamlogKey,
+  GetUltrabeamSettings, SaveUltrabeamSettings, TestUltrabeam, CompactDatabase, CheckHamlogKey, CompareRDASources,
   GetAntGeniusSettings, SaveAntGeniusSettings,
   GetTunerGeniusSettings, SaveTunerGeniusSettings,
   GetPSUSettings, SavePSUSettings,
@@ -1854,6 +1854,20 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
   const [rdaUseCurrent, setRdaUseCurrent] = useState(true);
   const [rdaMsg, setRdaMsg] = useState<string | null>(null);
   useEffect(() => { RDADatabaseCount().then((n) => setRdaCount(Number(n) || 0)).catch(() => {}); }, []);
+  // Measuring the two sources against each other, before deciding which wins.
+  //
+  // Read-only on purpose: the question "is HAMLOG's district independent
+  // evidence or a copy of the same database?" has to be answered with the
+  // operator's own log, and answering it must not modify that log.
+  const [rdaCmp, setRdaCmp] = useState<any>(null);
+  const [rdaCmpBusy, setRdaCmpBusy] = useState(false);
+  const runRDACompare = async () => {
+    setRdaCmpBusy(true); setRdaCmp(null);
+    try { setRdaCmp(await CompareRDASources()); }
+    catch (e: any) { setRdaMsg(String(e?.message ?? e)); }
+    finally { setRdaCmpBusy(false); }
+  };
+
   const runRDABackfill = async () => {
     setRdaBusy(true); setRdaMsg(null);
     try {
@@ -7220,6 +7234,56 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
           </div>
           <p className="text-[11px] text-muted-foreground">{t('rda.neverOverwrites')}</p>
         </div>
+
+        {/* Comparing the two sources. Read-only: it answers a question about the
+            log without touching it. */}
+        <div className="rounded-md border border-border p-3 space-y-2">
+          <div className="text-xs font-medium">{t('rda.cmpTitle')}</div>
+          <div className="flex items-center gap-3">
+            <Button size="sm" variant="secondary" onClick={runRDACompare} disabled={rdaCmpBusy}>
+              {rdaCmpBusy ? <Loader2 className="size-3.5 animate-spin mr-1.5" /> : null}
+              {t('rda.cmpRun')}
+            </Button>
+            {rdaCmp && (
+              <span className="text-xs text-muted-foreground">
+                {t('rda.cmpDone', {
+                  s: rdaCmp.scanned ?? 0, a: rdaCmp.agree ?? 0, d: rdaCmp.disagree ?? 0,
+                  l: rdaCmp.only_log ?? 0, b: rdaCmp.only_db ?? 0,
+                })}
+              </span>
+            )}
+          </div>
+          {/* The disagreements themselves. A count alone would say "there is a
+              problem" and leave the operator with no way to look at it. */}
+          {rdaCmp?.conflicts?.length > 0 && (
+            <div className="max-h-56 overflow-auto rounded border border-border">
+              <table className="w-full text-[11px]">
+                <thead className="sticky top-0 bg-card text-left text-muted-foreground border-b border-border">
+                  <tr>
+                    <th className="py-1 px-2 font-medium">{t('rda.cmpCall')}</th>
+                    <th className="py-1 pr-2 font-medium">{t('rda.cmpDate')}</th>
+                    <th className="py-1 pr-2 font-medium">{t('rda.cmpFromLog')}</th>
+                    <th className="py-1 pr-2 font-medium">{t('rda.cmpFromDb')}</th>
+                    <th className="py-1 pr-2 font-medium">{t('rda.cmpKind')}</th>
+                  </tr>
+                </thead>
+                <tbody>
+                  {rdaCmp.conflicts.map((c: any) => (
+                    <tr key={c.qso_id} className="border-b border-border/30">
+                      <td className="py-1 px-2 font-mono font-semibold">{c.callsign}</td>
+                      <td className="py-1 pr-2 font-mono">{c.date}</td>
+                      <td className="py-1 pr-2 font-mono">{c.from_log}{c.confirmed ? ' ✓' : ''}</td>
+                      <td className="py-1 pr-2 font-mono">{c.from_db}</td>
+                      <td className="py-1 pr-2 text-muted-foreground">
+                        {c.dated ? t('rda.cmpDated') : t('rda.cmpCurrent')}
+                      </td>
+                    </tr>
+                  ))}
+                </tbody>
+              </table>
+            </div>
+          )}
+        </div>
       </div>
     );
   }