feat: Kenwood CAT over a network serial bridge

The same ASCII stream over a socket instead of a wire: ser2net, an
Ethernet-serial adapter, a Raspberry Pi at the radio. One TCP transport
presented as a serial port, so the backend keeps a single code path.

Explicitly NOT the radio's own RJ45. A TS-890 or TS-990 speaks Kenwood's
KNS/ARCP there — session, authentication, a different protocol — and that needs
one of those radios in hand to write honestly. The setting's help text says so,
because an operator who plugs in their TS-890's Ethernet port and types its
address deserves to learn that from the interface rather than from silence.

Two details that decide whether this is usable or maddening:

  - A read deadline expiring is reported as "no data yet", not as an error. It
    is exactly what a serial read timeout means to the caller; as an error it
    would make ask() abandon a rig that is merely thinking.
  - The log line names what was connected to. "connected on  @ 0 baud" after a
    network connect sends someone hunting a serial fault that does not exist.

The host wins over the COM port when both are filled: it is the more deliberate
setting, and silently preferring the wire leaves someone staring at an address
they typed and a radio that never answers.

frontend/src/components/SettingsModal.tsx

@@ -1095,7 +1095,7 @@ 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, 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,
     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,
@@ -2563,6 +2563,15 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                 </Select>
                 <span className="text-xs text-muted-foreground">{t('cat.kenwoodBaudHint')}</span>
               </div>
+              <div className="space-y-1">
+                <Label>{t('cat.kenwoodHost')}</Label>
+                <Input
+                  value={catCfg.kenwood_host || ''}
+                  placeholder="192.168.1.50:4532"
+                  onChange={(e) => setCatCfg((s) => ({ ...s, kenwood_host: e.target.value }))}
+                />
+                <span className="text-xs text-muted-foreground">{t('cat.kenwoodHostHint')}</span>
+              </div>
             </>
             )}
             {catCfg.backend === 'icom' && (