fix Windows locale: force decimal dot in JSON state message

Format() used the system locale decimal separator, so on Russian/Eastern
European Windows smeter_dbm and float fields got commas ("−87,5"), producing
invalid JSON that JS silently dropped — breaking S-meter and frequency display.

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

WebServer.pas

@@ -1168,7 +1168,11 @@ function TWebServer.BuildStateJson: string;
 const
   MODE_N: array[0..7] of string =
     ('LSB','USB','DSB','CWL','CWU','FM','AM','SAM');
+var
+  FS: TFormatSettings;
 begin
+  FS := DefaultFormatSettings;
+  FS.DecimalSeparator := '.';
   FStateLock.Enter;
   try
     Result := Format(
@@ -1204,7 +1208,7 @@ begin
        FAttnIdx,
        BoolToStr(FTuning,  'true', 'false'),
        BoolToStr(FDuplex, 'true', 'false')
-      ]);
+      ], FS);
   finally
     FStateLock.Leave;
   end;