fix(elecraft): the ATU is switch 19, not 20

SWT20 was written from memory of the reference rather than from Table 7,
and 20 is not an ATU switch at all — it would have pressed something else
on a real K3. An operator read the table out for us: switch 19 is the ATU
row, TAP for a tuning cycle and HOLD for tuner in line or bypassed.

Adds the HOLD as its own ATU button, because on the radio they are two
different things: tuning is a cycle you start, bypassing is a state you
leave it in.

Power takes 0-110 W on an Elecraft, per the reference — a K3 makes a
little over its rated output — while a Kenwood keeps 200.

frontend/src/components/ElecraftPanel.tsx

@@ -4,7 +4,7 @@ import {
   GetKenwoodState, RefreshKenwood, SetKenwoodPower, SetKenwoodAFGain, SetKenwoodTX, TuneKenwoodATU,
   SetKenwoodRFGain, SetKenwoodMicGain, SetKenwoodSquelch, SetKenwoodPreamp, SetKenwoodAtt,
   SetKenwoodNB, SetKenwoodNR, SetKenwoodAGC, SetKenwoodFilter, SetKenwoodAntenna,
-  SetKenwoodRIT, SetKenwoodXIT, ClearKenwoodRIT, SetKenwoodKeySpeed,
+  SetKenwoodRIT, SetKenwoodXIT, ClearKenwoodRIT, SetKenwoodKeySpeed, ToggleKenwoodATU,
   GetCATState,
 } from '../../wailsjs/go/main/App';
 import { cn } from '@/lib/utils';
@@ -195,6 +195,15 @@ export function ElecraftPanel({ onReportRST }: { onReportRST?: (rst: string) =>
           className="flex-1 px-3 py-2.5 rounded-lg text-sm font-extrabold tracking-wide border-2 border-warning text-warning bg-card hover:bg-warning-muted transition-all disabled:opacity-30">
           <Activity className="size-4 inline mr-1 -mt-0.5" /> TUNE
         </button>
+        {/* The HOLD of the same switch: tuner in line or bypassed. Two buttons
+            because they are two things on the radio — tuning is a cycle you
+            start, bypassing is a state you leave it in. */}
+        <button type="button" disabled={off}
+          onClick={() => ToggleKenwoodATU().catch(setErrMsg)}
+          title={t('k3.atuHint')}
+          className="px-3 py-2.5 rounded-lg text-sm font-extrabold tracking-wide border-2 border-border text-muted-foreground bg-card hover:bg-muted transition-all disabled:opacity-30">
+          ATU
+        </button>
       </div>
 
       {/* Levels — two columns, so a slider stays beside the label it belongs to