feat: a typed watering hole carries its mode; Yaesu RTTY sideband

His log settles the 28.074 case: "SetCATFrequency 28.074 MHz" and the
state still reads mode=USB — nothing sent a mode, so the rig simply
stayed where it was. A spot click has always carried one; a frequency
typed by hand carried none. It now uses the same table and the same
tolerance as a spot (±3 kHz of a known FT8/FT4/JS8 frequency), and only
towards the digital modes: tuning AWAY from one leaves the mode alone,
because there the frequency says nothing about what the operator means.

RTTY on Yaesu is a choice the log cannot make: ADIF records "RTTY" and
the rig has MD06 (RTTY-L) and MD09 (RTTY-U). The older lower sideband
stays the default and a station whose FSK controller wants the upper one
says so once in Settings → CAT.

frontend/src/components/SettingsModal.tsx

@@ -1628,7 +1628,7 @@ function SettingsModalImpl({ onClose, onSaved, initialSection, onMainPaneChanged
   const [icomCustom, setIcomCustom] = useState(false);
   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, flex_dvk_dax: false,
-    yaesu_port: '', yaesu_baud: 38400, yaesu_low_lines: false, kenwood_low_lines: false, kenwood_port: '', kenwood_baud: 9600, kenwood_host: '', kenwood_link: 'usb', kenwood_data_mode: 'usb', xiegu_port: '', xiegu_baud: 19200, xiegu_addr: 0x70, xiegu_ptt_line: '',
+    yaesu_port: '', yaesu_baud: 38400, yaesu_low_lines: false, yaesu_rtty_usb: false, kenwood_low_lines: false, kenwood_port: '', kenwood_baud: 9600, kenwood_host: '', kenwood_link: 'usb', kenwood_data_mode: 'usb', 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, offset_on: false, offset_hz: 0,
     digital_default: 'FT8', share_enabled: false, share_port: 4532, share_proto: 'rigctl', share_tci_port: 40001,
@@ -3532,6 +3532,17 @@ function SettingsModalImpl({ onClose, onSaved, initialSection, onMainPaneChanged
                 </label>
                 <span className="text-xs text-muted-foreground">{t('cat.lowerLinesHint')}</span>
               </div>
+              {/* ADIF records "RTTY" and nothing more, and the rig has two
+                  sidebands to put it on — so this is the operator's answer, not
+                  something the log can supply. */}
+              <div className="col-span-2 space-y-1">
+                <label className="flex items-center gap-2 text-xs cursor-pointer">
+                  <Checkbox checked={!!(catCfg as any).yaesu_rtty_usb}
+                    onCheckedChange={(c) => setCatCfg((s) => ({ ...s, yaesu_rtty_usb: !!c } as any))} />
+                  {t('cat.yaesuRttyUsb')}
+                </label>
+                <span className="text-xs text-muted-foreground">{t('cat.yaesuRttyUsbHint')}</span>
+              </div>
             </>
             )}
             {['icom', 'xiegu', 'kenwood', 'elecraft'].includes(catCfg.backend) && (