fix(udp): edge-detect the decoder's DX Call stream

WSJT-X restates the same DX Call in every Status packet, and a spot clicked
in OpsLog put that call into the last-UDP memory (deliberately, for another
case) — so the guard let the decoder's next restatement straight through and
the entry snapped back to old news. The decoder's stream now only counts when
its value CHANGES; a clear over there resets the edge so re-selecting the
same station still lands. Explicit picks (N1MM, remote, spot relays) are
untouched.

frontend/src/App.tsx

@@ -2667,6 +2667,11 @@ export default function App() {
   // "the field still shows the previous broadcast" (safe to update) from "the
   // user has typed a different call" (must not clobber).
   const lastUdpCallRef = useRef('');
+  // Edge detection for the DECODER'S stream: WSJT-X/JTDX/MSHV re-broadcast the
+  // same DX Call in every Status packet, seconds apart, forever. Applying each
+  // one meant a spot clicked in OpsLog was overwritten moments later by the
+  // decoder restating old news. Only a CHANGE in this stream is an event.
+  const lastWsjtEdgeRef = useRef('');
 
   // When the entered callsign turns out to be worked-before, jump to the
   // Worked-before tab so the history is front-and-centre. Only once per call,
@@ -3796,6 +3801,13 @@ export default function App() {
       // Anything that isn't WSJT-X (N1MM, ADIF, a panadapter/cluster click relayed
       // over UDP…) is an explicit pick → force it over an existing call.
       const force = String(p?.service ?? '').toLowerCase() !== 'wsjt';
+      if (!force) {
+        // The decoder's stream: same value as last time = no edge = no update.
+        // Only a changed DX Call is the operator doing something over there.
+        const upper = String(p?.call ?? '').trim().toUpperCase();
+        if (upper && upper === lastWsjtEdgeRef.current) return;
+        lastWsjtEdgeRef.current = upper;
+      }
       // External app moved to a new station → fresh recording for the new target.
       if (applyUdpCall(p?.call, force)) restartRecordingForNewTarget(String(p?.call ?? ''));
     });
@@ -3806,6 +3818,9 @@ export default function App() {
     // Only when something is actually in the entry, so an idle digital app doesn't
     // wipe a call being typed by hand.
     const unsubClear = EventsOn('udp:clear_call', () => {
+      // The decoder cleared its DX Call: the next call it announces — even the
+      // same one re-selected — is a fresh edge.
+      lastWsjtEdgeRef.current = '';
       if (callsignRef.current?.value?.trim() || callsign.trim()) resetEntry();
     });
     // Clicked one of OpsLog's spots on the FlexRadio panadapter → fill the call