Phase 5: fill SetBand as a real band-by-index command

SetBand was a skeleton (FCurrentBand := Idx + Changed), leaving BandUp/BandDown
non-functional. Fill it with the band-change logic (XVTR-exit + persist + restore)
that WebAdapter.SyncBand had inline; SyncBand now just calls FController.SetBand.
Gives CAT band up/down/by-index a controller command to call (Phase 4) instead of
routing through the web adapter.

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

RadioController.pas

@@ -1545,7 +1545,20 @@ begin
 end;
 
 procedure TRadioController.SetBand(Idx: Integer);
-begin FCurrentBand := Idx; Changed(rfBand); { TODO wiring: RestoreBand } end;
+// Смена HF-диапазона по индексу (web/CAT/BandUp-Down). Выход из XVTR (если активен),
+// персист покидаемого, restore нового. Полная команда — не трогает UI.
+begin
+  if (Idx < 0) or (Idx >= CFG_BAND_COUNT) then Exit;
+  if FCurrentXvtr >= 0 then
+  begin
+    DeactivateXvtr;                // выход в HF (HF-возврат внутри)
+    if Idx = FCurrentBand then Exit;
+  end
+  else if Idx = FCurrentBand then Exit;
+  if FDevConnected then begin SaveCurrentBand; FSettings.Save; end;
+  FCurrentBand := Idx;
+  RestoreBand(Idx);
+end;
 
 procedure TRadioController.BandUp;
 begin if FCurrentBand < CFG_BAND_COUNT - 1 then SetBand(FCurrentBand + 1); end;