fix(entry): stop stray QSO logging from freq-Enter and bare-number DX call

Two ways a bogus contact could be logged:
- Enter in the TX/RX frequency field bubbled to the entry container's
  onKeyDown, which calls save() on Enter-in-an-input. Typing a frequency and
  pressing Enter tuned the rig AND logged the QSO. The freq fields now
  stopPropagation so Enter only tunes.
- WSJT-X/JTDX broadcasts its DX-call field over UDP; when it holds a bare
  number ("1"), applyUdpCall dropped it into the callsign, and the next Enter
  logged a QSO with callsign "1". Reject any DX call with no letter.

frontend/src/App.tsx

@@ -2607,6 +2607,11 @@ export default function App() {
     const applyUdpCall = (raw: string, force = false): boolean => {
       const call = String(raw ?? '').trim();
       if (!call) return false;
+      // A callsign always has at least one letter. WSJT-X/JTDX (and relayed spot
+      // clicks) sometimes broadcast a bare number in the DX-call field — famously
+      // "1" — which would land in the entry and, on the next Enter, log a QSO
+      // with callsign "1". Reject anything with no letter.
+      if (!/[a-z]/i.test(call)) return false;
       const upper = call.toUpperCase();
       const current = callsignValRef.current.trim().toUpperCase();
       const prev = lastUdpCallRef.current;
@@ -4355,7 +4360,7 @@ export default function App() {
         title={catState.connected ? t('field.freqTuneHint') : undefined}
         onFocus={() => setFreqFocused(true)}
         onBlur={() => setFreqFocused(false)}
-        onKeyDown={(e) => { if (e.key === 'Enter') tuneRadioTo(freqMhz); }}
+        onKeyDown={(e) => { if (e.key === 'Enter') { e.preventDefault(); e.stopPropagation(); tuneRadioTo(freqMhz); } }}
         onChange={(e) => { setFreqMhz(e.target.value); noteManualEdit(); const b = bandForMHz(parseFloat(e.target.value)); if (b) setBand(b); }}
       />
     </div>
@@ -4370,7 +4375,7 @@ export default function App() {
         title={catState.connected ? t('field.freqTuneHint') : undefined}
         onFocus={() => setFreqFocused(true)}
         onBlur={() => setFreqFocused(false)}
-        onKeyDown={(e) => { if (e.key === 'Enter') tuneRadioTo(rxFreqMhz); }}
+        onKeyDown={(e) => { if (e.key === 'Enter') { e.preventDefault(); e.stopPropagation(); tuneRadioTo(rxFreqMhz); } }}
         onChange={(e) => { setRxFreqMhz(e.target.value); noteManualEdit(); const rb = bandForMHz(parseFloat(e.target.value)); if (rb) setBandRx(rb); }}
         className={cn('font-mono', catState.split && 'bg-danger-muted/40 border-danger-border focus:bg-card')}
       />