feat(udp): spot clicks switch the decoder's mode

Configure (message 15), mode field only, every other field sent as
no-change: an FT4 spot clicked while WSJT-X sits in FT8 lands the operator
ready to decode instead of staring at gibberish. Sent to every instance
heard this session — one already in the right mode treats it as a no-op —
and only for modes the decoder actually speaks (FT8/FT4/JT65/JT9/MSK144/
Q65/FST4); CW and SSB are none of its business. Toggle in the Connections
panel, on by default.

frontend/src/components/UDPIntegrationsPanel.tsx

@@ -2,7 +2,7 @@ import React, { useCallback, useEffect, useState } from 'react';
 import { Plus, Trash2, Edit2, RefreshCcw, ArrowDownToLine, ArrowUpFromLine } from 'lucide-react';
 import {
   ListUDPIntegrations, SaveUDPIntegration, DeleteUDPIntegration, ReloadUDPIntegrations,
-  GetWsjtHighlight, SetWsjtHighlight,
+  GetWsjtHighlight, SetWsjtHighlight, GetWsjtFollowMode, SetWsjtFollowMode,
 } from '../../wailsjs/go/main/App';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
@@ -160,7 +160,11 @@ type Props = { onError: (msg: string) => void };
 
 export function UDPIntegrationsPanel({ onError }: Props) {
   const [highlightOn, setHighlightOn] = useState(false);
-  useEffect(() => { GetWsjtHighlight().then((v) => setHighlightOn(!!v)).catch(() => {}); }, []);
+  const [followMode, setFollowMode] = useState(true);
+  useEffect(() => {
+    GetWsjtHighlight().then((v) => setHighlightOn(!!v)).catch(() => {});
+    GetWsjtFollowMode().then((v) => setFollowMode(!!v)).catch(() => {});
+  }, []);
   const { t } = useI18n();
   const [items, setItems] = useState<UDPConfig[]>([]);
   const [loading, setLoading] = useState(true);
@@ -242,6 +246,14 @@ export function UDPIntegrationsPanel({ onError }: Props) {
           <span className="block text-[11px] text-muted-foreground">{t('udpp.highlightHint')}</span>
         </span>
       </label>
+      <label className="flex items-start gap-2 text-sm cursor-pointer max-w-2xl">
+        <Checkbox checked={followMode}
+          onCheckedChange={(c) => { setFollowMode(!!c); void SetWsjtFollowMode(!!c); }} />
+        <span>
+          {t('udpp.followMode')}
+          <span className="block text-[11px] text-muted-foreground">{t('udpp.followModeHint')}</span>
+        </span>
+      </label>
       <Section
         title={t('udpp.inboundTitle')}
         icon={<ArrowDownToLine className="size-4" />}