feat: amplifier band-follow — answer the amp's frequency polls

On its CAT/AUX connector an ACOM is the MASTER: it polls a transceiver and
changes band from the reply, so nothing can be pushed to it. internal/catemu
answers those polls on a second serial port, in ACOM command set 5 (Kenwood /
Elecraft): FA;, FB;, IF; and ID;, and nothing else — a wrong-length reply is
worse than none, it desynchronises the amp's parser for the following poll.

Also offered for SPE. Some amps do not poll at all but read the radio↔PC CAT
line in parallel; those never hear a responder, so an optional unprompted send
(500/1000 ms) covers them.

The TX frequency is what is sent: in split the amp must be tuned where we
transmit. Frame lengths are pinned by tests — the failure they guard against is
silent and only shows up as an amp that mistunes.

Untested on hardware.

frontend/src/components/SettingsModal.tsx

@@ -627,7 +627,10 @@ function ADIFMonitorPanel() {
 }
 
 // AmpUI mirrors the backend AmpConfig — one configured amplifier.
-type AmpUI = { id: string; name: string; enabled: boolean; type: string; transport: string; host: string; port: number; com_port: string; baud: number };
+type AmpUI = { id: string; name: string; enabled: boolean; type: string; transport: string; host: string; port: number; com_port: string; baud: number;
+  // Band-follow (ACOM): a SECOND serial port on which OpsLog answers the amp's
+  // frequency polls, pretending to be a Kenwood-format transceiver.
+  freq_out?: boolean; freq_com_port?: string; freq_baud?: number; freq_broadcast_ms?: number };
 
 // RelayAutoPanel configures automatic control of the Station Control relay boards
 // from the rig's frequency / band (PstRotator-style). Each relay carries one rule:
@@ -2811,6 +2814,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
     });
     const addAmp = () => setAmps((l) => [...l, {
       id: '', name: '', enabled: true, type: 'spe13', transport: 'tcp', host: '', port: 9008, com_port: '', baud: 115200,
+      freq_out: false, freq_com_port: '', freq_baud: 9600, freq_broadcast_ms: 0,
     }]);
     return (
       <>
@@ -2927,6 +2931,62 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                   </div>
                 )}
 
+                {/* Band-follow, for any amp that takes its band from a transceiver
+                    CAT link (ACOM and SPE both do) — never PowerGenius, which is
+                    driven over its network protocol. A SECOND serial port,
+                    separate from the metering one above. */}
+                {!isPGXL && (
+                  <div className="border-t border-border/60 pt-3 space-y-3">
+                    <label className="flex items-center gap-2 text-sm cursor-pointer">
+                      <Checkbox
+                        checked={!!amp.freq_out}
+                        onCheckedChange={(c) => patchAmp(i, { freq_out: !!c })}
+                      />
+                      {t('amp.freqOut')}
+                    </label>
+                    {amp.freq_out && (
+                      <>
+                        <div className="grid grid-cols-3 gap-3">
+                          <div className="space-y-1 col-span-2">
+                            <Label>{t('amp.freqPort')}</Label>
+                            <div className="flex items-center gap-2">
+                              <Select value={amp.freq_com_port || '_'} onValueChange={(v) => patchAmp(i, { freq_com_port: v === '_' ? '' : v })}>
+                                <SelectTrigger className="h-9 flex-1"><SelectValue placeholder="— COM —" /></SelectTrigger>
+                                <SelectContent>
+                                  {wkPorts.length === 0 && <SelectItem value="_" disabled>No ports found</SelectItem>}
+                                  {wkPorts.map((p) => <SelectItem key={p} value={p}>{p}</SelectItem>)}
+                                </SelectContent>
+                              </Select>
+                              <Button size="sm" variant="outline" className="h-9" onClick={() => ListSerialPorts().then((p) => setWkPorts((p ?? []) as string[])).catch(() => {})}>
+                                <ArrowDown className="size-3.5 rotate-90" />
+                              </Button>
+                            </div>
+                          </div>
+                          <div className="space-y-1">
+                            <Label>Baud</Label>
+                            <Input type="number" min={1200} value={amp.freq_baud ?? 9600}
+                              onChange={(e) => patchAmp(i, { freq_baud: parseInt(e.target.value) || 9600 })} className="font-mono" />
+                          </div>
+                        </div>
+                        <div className="grid grid-cols-3 gap-3">
+                          <div className="space-y-1 col-span-2">
+                            <Label>{t('amp.freqBroadcast')}</Label>
+                            <Select value={String(amp.freq_broadcast_ms ?? 0)} onValueChange={(v) => patchAmp(i, { freq_broadcast_ms: parseInt(v) })}>
+                              <SelectTrigger className="h-9"><SelectValue /></SelectTrigger>
+                              <SelectContent>
+                                <SelectItem value="0">{t('amp.freqPollOnly')}</SelectItem>
+                                <SelectItem value="500">{t('amp.freqEvery', { ms: 500 })}</SelectItem>
+                                <SelectItem value="1000">{t('amp.freqEvery', { ms: 1000 })}</SelectItem>
+                              </SelectContent>
+                            </Select>
+                          </div>
+                        </div>
+                        <p className="text-[10px] text-muted-foreground">{t('amp.freqHint')}</p>
+                      </>
+                    )}
+                  </div>
+                )}
+
                 {amp.enabled && amp.id && <AmpStatusCard id={amp.id} />}
                 {!isPGXL && !isACOM && (
                   <p className="text-[10px] text-muted-foreground">