fix: serial CAT keyed the radio on connect; Xiegu can key on RTS/DTR

Reported on a Xiegu G90. With the blue programming cable — three wires, no
modem lines — CAT works perfectly. Behind a DE-19, which carries audio, CAT and
PTT, the radio went into transmit the moment OpsLog connected and stayed there.

Windows raises DTR and RTS when a serial port is opened, and the DE-19 reads
them as PTT; Xiegu's own documentation asks for both low. The CI-V backend has
dropped them since it was written, for the same reason on Icom rigs with USB
SEND mapped to a line. Xiegu, Yaesu and Kenwood did not. They do now.

The second half of the report: WSJT-X through OpsLog's rigctld decoded fine and
never transmitted. Nothing was broken in that chain — set_ptt reaches the
backend, which sends the CI-V PTT command, which a G90 ignores. That is why
Xiegu keys on a hardware line instead. The backend can now do that: Settings →
CAT → Xiegu → how the rig is keyed (CI-V / RTS / DTR).

Untested here — no G90 in reach. The operator who reported it offered to try.

frontend/src/components/SettingsModal.tsx

@@ -1136,7 +1136,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
   const [modeDraft, setModeDraft] = useState('');
   const [catCfg, setCatCfg] = useState<CATSettings>({
     enabled: false, backend: 'omnirig', omnirig_rig: 1, omnirig_vfo: '', flex_host: '', flex_port: 4992, flex_spots: false, flex_decode_spots: false, flex_decode_secs: 120,
-    yaesu_port: '', yaesu_baud: 38400, kenwood_port: '', kenwood_baud: 9600, kenwood_host: '', xiegu_port: '', xiegu_baud: 19200, xiegu_addr: 0x70,
+    yaesu_port: '', yaesu_baud: 38400, kenwood_port: '', kenwood_baud: 9600, kenwood_host: '', xiegu_port: '', xiegu_baud: 19200, xiegu_addr: 0x70, xiegu_ptt_line: '',
     icom_port: '', icom_baud: 115200, icom_addr: 0x98, icom_net_host: '', icom_net_user: '', icom_net_pass: '', icom_net_audio: false,
     tci_host: '', tci_port: 40001, tci_spots: false, poll_ms: 250, delay_ms: 0,
     digital_default: 'FT8', share_enabled: false, share_port: 4532,
@@ -2538,6 +2538,19 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                   }} />
                 <span className="text-xs text-muted-foreground">{t('cat.xieguAddrHint')}</span>
               </div>
+              <div className="space-y-1">
+                <Label>{t('cat.xieguPTTLine')}</Label>
+                <Select value={catCfg.xiegu_ptt_line || 'civ'}
+                  onValueChange={(v) => setCatCfg((s) => ({ ...s, xiegu_ptt_line: v === 'civ' ? '' : v }))}>
+                  <SelectTrigger><SelectValue /></SelectTrigger>
+                  <SelectContent>
+                    <SelectItem value="civ">{t('cat.xieguPTTCiv')}</SelectItem>
+                    <SelectItem value="rts">RTS</SelectItem>
+                    <SelectItem value="dtr">DTR</SelectItem>
+                  </SelectContent>
+                </Select>
+                <span className="text-xs text-muted-foreground">{t('cat.xieguPTTHint')}</span>
+              </div>
             </>
             )}
             {catCfg.backend === 'yaesu' && (