Fix FM step change from web UI; add FM grid to web spectrum/ruler

Web UI: stepSel.onchange now sends fm_step command to server when in FM
mode; option values stay in Hz so tuneStep works unchanged.
WebServer: add fm_step command handler and OnFMStep callback.
MainForm: wire WebOnFMStep -> SyncWebFMStep -> SetFMStep + SaveCurrentBand.

Web spectrum/ruler: vertical FM-step grid lines in drawSpec; FM-aligned
ticks and labels in drawRuler with auto label-density (short ticks between
labeled lines, measureText-based labelMult to prevent overlap).

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

MainForm.pas

@@ -564,6 +564,7 @@ type
     procedure WebOnFreqA(Hz: Double);
     procedure WebOnAttn(Idx: Integer);
     procedure WebOnTun(On_: Boolean);
+    procedure WebOnFMStep(Idx: Integer);
     procedure WebOnMic(Samples: PSingle; Count: Integer);
     procedure WebOnXvtrBand(Idx: Integer);
     // Synchronize-обёртки (выполняются в UI-потоке)
@@ -593,6 +594,7 @@ type
     procedure SyncWebFreqA;
     procedure SyncWebAttn;
     procedure SyncWebTun;
+    procedure SyncWebFMStep;
     // --- CAT callbacks -------------------------------------------------------
     function  CATGetVfoA:       Double;
     function  CATGetVfoB:       Double;
@@ -1257,6 +1259,7 @@ begin
   FWebServer.OnFreqA     := WebOnFreqA;
   FWebServer.OnAttn      := WebOnAttn;
   FWebServer.OnTun       := WebOnTun;
+  FWebServer.OnFMStep    := WebOnFMStep;
   FWebServer.OnWebMic    := WebOnMic;
   FWebServer.OnXvtrBand  := WebOnXvtrBand;
   if FWebEnabled then FWebServer.Start;
@@ -5851,6 +5854,19 @@ begin
   FNetwork.SetStepAtten(FAtten * 10);
 end;
 
+procedure TMainForm.WebOnFMStep(Idx: Integer);
+begin
+  FWebSyncInt := Idx;
+  FWebSyncM   := SyncWebFMStep;
+  TThread.Synchronize(nil, FWebSyncM);
+end;
+
+procedure TMainForm.SyncWebFMStep;
+begin
+  SetFMStep(Max(0, Min(FM_STEP_COUNT - 1, FWebSyncInt)));
+  SaveCurrentBand;
+end;
+
 procedure TMainForm.WebOnTun(On_: Boolean);
 begin
   FWebSyncBool := On_;