web: fix A=B/SWAP VFO ops, wire attenuator, TUN button, FNP filter overlay

- Add freq_a command: sets VFO A regardless of active VFO; fixes A=B and
  SWAP when VFO B is active (prev. sent freq_b which overwrote wrong VFO)
- Add pend("freq") to B=A / A=B / SWAP to guard local display during round-trip
- RX step attenuator: HPSDRNetwork.SetStepAtten → bytes 1442/1443 in HP packet;
  WebServer attn command; MainForm FAtten + SyncWebAttn; attSel.onchange in JS
- TUN mode: set_tun WebSocket command wired to ApplyTUN(); toneBtn toggles
  with on/off highlight and pending protection; tuning field in state JSON
- FNP button opens semi-transparent filter overlay (10 buttons, 5×2 grid)
  with mode-appropriate widths (SSB/CW/AM/FM); active filter highlighted;
  cur.filter_bw updated immediately on click so passband redraws within 50ms

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

HPSDRNetwork.pas

@@ -141,6 +141,7 @@ type
     FIsTransmitting: Boolean;
     FPAEnabled:      Boolean;
     FAlexEnabled:    Boolean;
+    FStepAtten:      Byte;     // ADC0 step attenuator 0-31 dB
 
     FSendLock: TCriticalSection;  // защита concurrent UDP sends
 
@@ -190,6 +191,7 @@ type
                             DriveLevel: Byte = 100);
     procedure UpdateState(RXFreqHz, TXFreqHz: Double; DriveLevel: Byte;
                           Transmitting, PAEnabled, AlexEnabled: Boolean);
+    procedure SetStepAtten(dB: Byte);
     procedure SendFullHP;
     procedure SendDDCAudio(const LeftRight: array of SmallInt);
     procedure SendDUCIQ(const IData, QData: array of Integer);
@@ -1208,6 +1210,13 @@ begin
   FAlexEnabled    := AlexEnabled;
 end;
 
+procedure THPSDRNetwork.SetStepAtten(dB: Byte);
+begin
+  if dB > 31 then dB := 31;
+  FStepAtten := dB;
+  if FConnected then SendFullHP;
+end;
+
 procedure THPSDRNetwork.SendFullHP;
 var
   Buf:      array[0..1443] of Byte;
@@ -1274,8 +1283,8 @@ begin
 
   // Step attenuators ADC0/ADC1 (bytes 1443/1442)
   // 0 dB = 0, max 31 dB
-  Buf[1443] := 0;   // ADC0 attenuation
-  Buf[1442] := 0;   // ADC1 attenuation
+  Buf[1443] := FStepAtten;  // ADC0 attenuation
+  Buf[1442] := FStepAtten;  // ADC1 attenuation
 
   DoSendTo(FSocket, Buf, SizeOf(Buf), FDevice.IPAddress, FPortHPFromPC);
 end;