fix web TX spectrum position in CTUN mode

Add 'dup' field to state JSON (FDisplayDuplex). In CTUN+non-DUP TX,
use vfo_a_hz as spectrum center so the TX signal renders at the VFO
position instead of the display center. In CTUN+DUP TX, keep center_hz
so the RX spectrum stays correctly positioned at the filter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

MainForm.pas

@@ -2776,7 +2776,7 @@ begin
       FCenterFreq, FFilter,
       FVfoB, FActiveVfo,
       FTransmitting, TrkDrive.Position,
-      FAtten, FTuning);
+      FAtten, FTuning, FDisplayDuplex);
 end;
 
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