fix(dvk): a transiently blank mode no longer kills the auto-CQ loop

Over the network the rig's mode read fails now and then and the CAT push
blanks the mode for a poll or two; the auto-CQ loop treated that blank as
'not a phone mode' and stopped after the first call. Only a KNOWN non-phone
mode stops the loop now.

frontend/src/App.tsx

@@ -1747,7 +1747,11 @@ export default function App() {
     dvkAutoCqSlotRef.current = slot;
     const sleep = (ms: number) => new Promise((r) => window.setTimeout(r, ms));
     while (dvkAutoCqSlotRef.current === slot && gen === dvkAutoCqGenRef.current && dvkActiveRef.current) {
-      if (!isPhoneMode(modeRef.current)) { stopDvkAutoCq(); break; }
+      // An EMPTY mode is not a mode change: over the network the rig's mode
+      // read fails transiently and the CAT push blanks the field for a poll or
+      // two — killing the auto-CQ loop mid-run for nothing. Only a known
+      // non-phone mode stops the loop.
+      if (modeRef.current && !isPhoneMode(modeRef.current)) { stopDvkAutoCq(); break; }
       await DVKPlay(slot).catch(() => {});
       await sleep(300);                        // let playback flip "playing" true
       let guard = 0;                           // then wait for it to finish (cap ~90 s)