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.

frontend/wailsjs/go/main/App.js

@@ -1346,6 +1346,10 @@ export function IcomSetSplit(arg1) {
   return window['go']['main']['App']['IcomSetSplit'](arg1);
 }
 
+export function IcomSetSplitOffset(arg1) {
+  return window['go']['main']['App']['IcomSetSplitOffset'](arg1);
+}
+
 export function IcomSetSquelch(arg1) {
   return window['go']['main']['App']['IcomSetSquelch'](arg1);
 }