Phase 3 (batch 2): route AGC mode through controller command

TRadioController.SetAGCMode now owns the index->WDSP mapping, applies to
the DSP engine, and updates the band cache, then fires OnStateChanged.
OnControllerState restyles the AGC button group. BtnAGCModeClick,
OnVfoOverlayAGCChange, and SyncWebAGC all call the command.

Fixes a latent web bug: SyncWebAGC used TWDSPAGCMode(index) directly,
mapping the UI index (0=FAST..4=OFF) onto the wrong enum (0=OFF...), so
web AGC selections set the wrong DSP mode. The button/overlay path used
the correct AGCModes[] array; unifying on the command applies that to web
too. Buttons/overlay behavior unchanged.

Builds clean (cocoa).

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

RadioController.pas

@@ -382,7 +382,17 @@ procedure TRadioController.SetFilterBW(BWHz: Integer);
 begin FFilterBW := BWHz; Changed(rfFilterBW); { TODO wiring } end;
 
 procedure TRadioController.SetAGCMode(M: Integer);
-begin FAGCMode := M; Changed(rfAGCMode); { TODO wiring: FDSPEngine.SetAGC } end;
+const
+  // UI-индекс 0..4 (FAST/MED/SLOW/LONG/OFF) -> WDSP enum. Единый маппинг для
+  // всех фронтендов (раньше web ошибочно кастовал индекс прямо в enum).
+  AGCModes: array[0..4] of TWDSPAGCMode = (agcFast, agcMedium, agcSlow, agcLong, agcOff);
+begin
+  if (M < 0) or (M > 4) then Exit;
+  FAGCMode := M;
+  if FWDSPReady and Assigned(FDSPEngine) then FDSPEngine.SetAGC(AGCModes[M], 50.0);
+  FBandCache[FCurrentBand].AGCMode := FAGCMode;
+  Changed(rfAGCMode);
+end;
 
 procedure TRadioController.SetAGCTop(DB: Integer);
 begin FAGCTop := DB; Changed(rfAGCTop); { TODO wiring } end;