feat(decodes): say when the decoder's band is not the rig's

A decoding application that loses its CAT link keeps announcing the last
dial frequency it knew, and every decode after that carries a stale
band. Nothing downstream can tell: the entity verdicts, the band filter
and the FT map all believe it. Seen for real — MSHV kept saying 80 m,
and Korea read as a NEW BAND because on 80 m it would have been.

Said, not decided. Taking the rig's band instead would be wrong for
anyone decoding a second receiver on another band, and a warning costs
that setup nothing but a line to read past. Shown only while CAT is
actually connected: an empty band means there is nothing to compare
with, never that the rig is on no band. Opens 0.27.9.

frontend/src/App.tsx

@@ -6328,6 +6328,9 @@ export default function App() {
       txState={txState}
       txStates={txStates}
       spotStatus={spotStatus as any}
+      // Only while CAT is actually connected: an empty band means "nothing to
+      // compare with", never "the rig is on no band".
+      rigBand={catState.connected ? (catState.band || '') : ''}
       myCall={station.callsign}
       // A click ANSWERS the station: it hands the decode back to WSJT-X/MSHV as
       // a Reply, which is the same thing as double-clicking the line in their