feat(k3): a mode row on the Elecraft console — CW/USB/LSB/DATA/DATA-RTTY

The report: switching the K3 to a data mode 'does not work for FT8 — the K3
does not go into DATA'. The mode digit is only half the answer on that radio:
MD6 keeps whatever DT submode the previous session left, and with FSK D still
armed the rig keys FT8 with no rear-audio modulation.

The panel buttons say the whole thing: DATA is MD6+DT0 (DATA A, the soundcard
path), DATA RTTY is MD6+DT2 (FSK D). The DT submode is also read on the
settings beat — Elecraft only, a plain Kenwood would '?;' it — so the active
DATA button follows what the rig is actually in, front-panel changes included.
A dedicated SetKenwoodPanelMode rather than the logger's SetMode: a panel
button is the operator saying exactly what the rig should do, not an ADIF mode
to be mapped through preferences.

app_k3.go

@@ -24,6 +24,12 @@ func (a *App) GetKenwoodState() cat.KenwoodTXState {
 	return st
 }
 
+// SetKenwoodPanelMode sets the operating mode from the panel's mode row —
+// CW / USB / LSB / DATA (soundcard, DT0) / RTTY (FSK D, DT2).
+func (a *App) SetKenwoodPanelMode(mode string) error {
+	return a.kenwoodPanelDo(func(k cat.KenwoodPanelController) error { return k.SetKenwoodPanelMode(mode) })
+}
+
 func (a *App) SetKenwoodPower(w int) error {
 	return a.kenwoodPanelDo(func(k cat.KenwoodPanelController) error { return k.SetKenwoodPower(w) })
 }