Fix mouse wheel always tuning VFO A regardless of active VFO

FormMouseWheel quantized from FVfoA and always called ApplyVfoA, so the
wheel tuned VFO A even when VFO B was active. Pre-existing bug (the wheel
handler never branched on FActiveVfo), surfaced while testing VFO B.

Quantize from ActiveVfoFreq and route to SetVfoB when B is active.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

MainForm.pas

@@ -6937,13 +6937,14 @@ begin
   Delta := 1;
   if WheelDelta < 0 then Delta := -1;
 
-  // Квантование (та же логика что в FreqDisplay.ChangeByDigit)
-  Base := (Round(FController.FVfoA) div Step) * Step;
+  // Квантование (та же логика что в FreqDisplay.ChangeByDigit) — от частоты
+  // АКТИВНОГО VFO, а не всегда VFO A.
+  Base := (ActiveVfoFreq div Step) * Step;
   if Delta > 0 then
     NewFreq := Base + Step
   else
   begin
-    if Round(FController.FVfoA) = Base then
+    if ActiveVfoFreq = Base then
       NewFreq := Base - Step
     else
       NewFreq := Base;   // снэп к нижней границе
@@ -6955,7 +6956,10 @@ begin
     NewFreq := Max(0, NewFreq)
   else
     NewFreq := Max(30000, Min(60000000, NewFreq));
-  ApplyVfoA(NewFreq);
+  if FController.FActiveVfo = 0 then
+    ApplyVfoA(NewFreq)
+  else
+    FController.SetVfoB(NewFreq);   // крутим активный VFO B
 
   Handled := True;
 end;