Align spectrum grid and ruler to FM tuning step

When FM mode is active, vertical grid lines and frequency labels snap to
FM channel boundaries (FM_STEP_HZ) instead of the fixed 8-division layout.
Grid and label density auto-adapt: lines thinner than 4px are thinned,
labels are spaced so they never overlap (short tick shown between labeled
lines). Grid cache invalidates on center-freq, step, or span changes.

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

MainForm.pas

@@ -2797,6 +2797,10 @@ begin
   FSpecView.SpanHz    := FSpanHz;
   FSpecView.Mode      := FMode;
   FSpecView.FilterBW  := FFilterBW;
+  if FMode = MODE_FM then
+    FSpecView.FMGridStepHz := FM_STEP_HZ[FFMStepIdx]
+  else
+    FSpecView.FMGridStepHz := 0;
   // TX overlay следует за активной TX-частотой (учитывает split). При CTUN
   // спектр TX будет отрисован на VFO, а не в центре дисплея.
   FSpecView.TXFreq    := ActiveTXFreqHz;
@@ -4805,6 +4809,8 @@ begin
   if FBtnFMSteps[Idx] <> nil then
     FBtnFMSteps[Idx].Active := True;
   if FWebServer <> nil then FWebServer.FMStepIdx := Idx;
+  if (FMode = MODE_FM) and (FSpecView <> nil) then
+    FSpecView.FMGridStepHz := FM_STEP_HZ[Idx];
 end;
 
 procedure TMainForm.BtnFMStepClick(Sender: TObject);