feat(icom): PTT and offset split at the operator's hand

The MOX and SPLIT controls lived at the bottom of the Transmit card, below
the fold on most layouts — a console whose PTT needs scrolling is not a
console. A strip under the mode row now carries the PTT and Split as
OFF/+1k/+5k/+10k, DXpedition style, on a new SetIcomSplitOffset that takes
the chosen TX offset (0 keeps the old CW/SSB convention). The mode-alignment
from the previous fix rides along on every engage.

app.go

@@ -14533,6 +14533,15 @@ func (a *App) IcomSetSplit(on bool) error {
 	return a.cat.IcomDo(func(ic cat.IcomController) error { return ic.SetIcomSplit(on) })
 }
 
+// IcomSetSplitOffset turns split on with a chosen TX offset (Hz) — the
+// console's +1/+5/+10 kHz buttons.
+func (a *App) IcomSetSplitOffset(hz int64) error {
+	if a.cat == nil {
+		return fmt.Errorf("cat not initialized")
+	}
+	return a.cat.IcomDo(func(ic cat.IcomController) error { return ic.SetIcomSplitOffset(true, hz) })
+}
+
 func (a *App) IcomSetAntenna(n int) error {
 	if a.cat == nil {
 		return fmt.Errorf("cat not initialized")