fix(sat): both antennas on both satellite slices

A slice has an rxant and a txant, and each pair belongs to that slice's
own band. Only two of the four were being set — the downlink's receive
antenna and the uplink's transmit one — so the downlink slice was left
with an empty txant. It never keys, so nothing was wrong on the air, but
the slice was half-configured: move transmit focus to it and the radio
uses whatever antenna it happened to be left on. Reported with XVTA on
2 m and XVTB on 70 cm, where the 70 cm slice showed one antenna and the
2 m slice showed two.

SatAntennas now takes all four, and the late-slice replay in
adoptSatSlice hands the uplink its own pair instead of putting the
transmit antenna on both of its ports.

internal/cat/flex.go

@@ -79,9 +79,13 @@ type Flex struct {
 	// turns up LATE. Arming, the antennas, the tone and the mode all happen
 	// before the radio has necessarily reported the slice it was asked to
 	// create; without this they were applied to an index of -1 and never again.
-	satUpMode       string
-	satRXAnt        string
-	satTXAnt        string
+	satUpMode string
+	// Both antennas of both slices: a slice has an rxant and a txant, and each
+	// pair belongs to that slice's own band.
+	satDownRX       string
+	satDownTX       string
+	satUpRX         string
+	satUpTX         string
 	satTone         float64
 	spotCall        map[int]string // spot index → callsign (to fill the call on a panadapter click)
 	spotMode        map[int]string // spot index → ADIF mode, so a click can also set the slice mode (SmartSDR tunes the spot's freq but not its mode)