Phase 3 (batch 5): route FM step through controller command

FM step is the only FM control exposed to another frontend (web:
SyncWebFMStep in, FWebServer.FMStepIdx out), so it's the FM param with
real cross-frontend value.

- TRadioController.SetFMStep clamps and fires OnStateChanged(rfFMStepIdx).
- OnControllerState updates the step dropdown, pushes FMStepIdx to the web
  server, and sets the FSpecView FM grid step (when in FM mode).
- MainForm.SetFMStep is now a thin wrapper over the command, so existing
  callers (band restore, dropdown select, web sync, XVTR restore) are
  unchanged.

FM squelch/CTCSS/repeater are intentionally left as-is: they are UI-only
(no web/CAT/daemon exposure) and already operate on controller state
fields, so routing them through commands adds ceremony without value for
the swappable-UI/daemon goals; CTCSS would also need CTCSS_TONES relocated
to a shared unit and repeater needs the network helpers (XvtrTranslate/
ActiveTXFreqHz) that belong to the heavy-command batch.

Builds clean (cocoa).

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

RadioController.pas

@@ -516,7 +516,11 @@ procedure TRadioController.SetTune(On_: Boolean);
 begin FTuning := On_; Changed(rfTuning); { TODO wiring: ApplyTUN } end;
 
 procedure TRadioController.SetFMStep(Idx: Integer);
-begin FFMStepIdx := Idx; Changed(rfFMStepIdx); { TODO wiring } end;
+begin
+  if Idx < 0 then Idx := 0 else if Idx > 3 then Idx := 3;   // FM_STEP_COUNT = 4
+  FFMStepIdx := Idx;
+  Changed(rfFMStepIdx);
+end;
 
 procedure TRadioController.SetFMCTCSS(On_: Boolean);
 begin FFMCTCSSOn := On_; Changed(rfFMCTCSS); { TODO wiring } end;