feat: CW through the Yaesu keyer — fifth CW engine

The radio has a keyer and a command to feed it (KY), so an FTDX10 needs no
WinKeyer and no second cable, exactly as the Icom CI-V and Flex CWX engines
already do. The rig keys with its own timing, which is why the spacing is right
where a PC keying a line through USB latency drifts.

Text is filtered to what the keyer can actually send: an unsupported byte does
not produce an error on a Yaesu, it can abort the whole buffer, so the rest of a
macro would vanish silently. It is then fed in 24-character pieces, waiting for
room between them — the rig DROPS what does not fit, again with no error, so a
contest CQ would lose its tail.

STOP is the honest gap. Yaesu documents no buffer-clear, so it drops the
transmitter (TX0) instead: nothing queued reaches the air, which is what Escape
means to an operator. It deliberately does NOT send "KY0;" — a plausible-looking
clear that the rig would read as the CHARACTER zero and transmit.

A test caught a real one on the way: tabs and newlines were dropped as
"unsupported", gluing the words either side together, so a macro written on two
lines went out as CQCQ. Whitespace now becomes a word gap before filtering.

Settings warn when the Yaesu keyer is selected without the Yaesu CAT backend —
otherwise it simply never keys, with nothing on screen saying why.

Send path follows the CAT reference and how Hamlib drives these rigs; NOT yet
verified on the air.

frontend/src/components/SettingsModal.tsx

@@ -3309,6 +3309,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                   <SelectItem value="winkeyer">{t('wk.engWinkeyer')}</SelectItem>
                   <SelectItem value="serial">{t('wk.engSerial')}</SelectItem>
                   <SelectItem value="icom">{t('wk.engIcom')}</SelectItem>
+                  <SelectItem value="yaesu">{t('wk.engYaesu')}</SelectItem>
                   <SelectItem value="flex">{t('wk.engFlex')}</SelectItem>
                   <SelectItem value="tci" disabled>{t('wk.engTci')}</SelectItem>
                 </SelectContent>
@@ -3324,6 +3325,12 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
             </label>
           </div>
 
+          {wk.engine === 'yaesu' && (!catCfg.enabled || catCfg.backend !== 'yaesu') && (
+            <p className="text-xs font-medium text-warning -mt-1 flex items-start gap-1.5">
+              <span aria-hidden>⚠</span>
+              <span>{t('wk.catWarnYaesu', { backend: catCfg.enabled ? (catCfg.backend || 'none') : 'disabled' })}</span>
+            </p>
+          )}
           {wk.engine === 'icom' ? (
             <>
               {(!catCfg.enabled || catCfg.backend !== 'icom') && (