feat(kpa): the band-follow is asked for, not assumed

Telling somebody's amplifier where to be is a write to their station, and
it now has a switch — the SAME switch an Acom uses. 'Keep the amplifier on
the radio's band' is one idea to an operator; that it needs a second
serial port and a rig emulator on one amplifier and a single ^BN on
another is our business, not a reason for two checkboxes.

On by default for a KPA, because an amplifier left on the wrong band is
the worse outcome of the two, and the operator who has wired it straight
to the radio turns it off once. The hint says exactly that case, since it
is the only one where a second voice telling the amplifier where to be is
unwelcome.

frontend/src/components/SettingsModal.tsx

@@ -4177,6 +4177,7 @@ function SettingsModalImpl({ onClose, onSaved, initialSection, onMainPaneChanged
     // could never connect.
     const applyType = (i: number, v: string) => patchAmp(i, {
       type: v,
+      freq_out: v.startsWith('kpa') ? true : amps[i].freq_out,
       transport: v === 'pgxl' ? 'tcp' : v === 'kpa500' ? 'serial' : amps[i].transport,
       baud: v.startsWith('acom') ? 9600 : v.startsWith('spe') ? 115200 : v.startsWith('kpa') ? 38400 : amps[i].baud,
       port: v === 'kpa1500' ? 1500 : amps[i].port,
@@ -4323,6 +4324,18 @@ function SettingsModalImpl({ onClose, onSaved, initialSection, onMainPaneChanged
                     Offering a second serial port and a transceiver emulator for
                     that would be a workaround for a problem this amplifier does
                     not have. */}
+                {isKPA && (
+                  <div className="border-t border-border/60 pt-3 space-y-1">
+                    <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.kpaBandFollow')}
+                    </label>
+                    <p className="text-[11px] text-muted-foreground">{t('amp.kpaBandFollowHint')}</p>
+                  </div>
+                )}
                 {!isPGXL && !isKPA && (
                   <div className="border-t border-border/60 pt-3 space-y-3">
                     <label className="flex items-center gap-2 text-sm cursor-pointer">