feat: serial-line CW keyer (DTR=CW / RTS=PTT) — SCU-17 / N1MM style

Adds a 4th CW keyer engine alongside K1EL WinKeyer, Icom CI-V and Flex CWX:
OpsLog bit-bangs the Morse itself on a serial control line (DTR keys CW, RTS
keys PTT — swappable), the hardware-keying method N1MM/WSJT use with a Yaesu
SCU-17 and generic interfaces. No WinKeyer chip needed.

- internal/winkeyer/serialcw.go: serialKeyer — worker goroutine, morse table,
  WPM + Farnsworth timing (live speed changes), abortable, PTT lead-in/tail/hang.
- winkeyer.Config gains Type ("k1el"|"serial") + CWKeyLine ("dtr"|"rts");
  Manager routes Connect/Send/Stop/SetSpeed/Backspace/Disconnect to it.
- app.go: keyWKCWLine setting; WinkeyerConnect sets Type from the engine.
- Settings -> CW Keyer: new "Serial port (DTR=CW/RTS=PTT)" engine with COM port,
  key-line selector, speed/Farnsworth/lead-in/tail/PTT. Macros, auto-CQ and
  <LOGQSO> reuse the WinKeyer path unchanged.

UNTESTED on hardware (no SCU-17 to hand): polarity assumes N1MM convention
(line HIGH = active). Weight not yet honoured; Farnsworth is.

frontend/src/App.tsx

@@ -2130,7 +2130,7 @@ export default function App() {
       setWkMacros((s.macros ?? []) as WKMacro[]);
       setWkEscClears(s.esc_clears_call !== false);
       setWkSendOnType(!!s.send_on_type);
-      setWkEngine(s.engine === 'icom' ? 'icom' : s.engine === 'flex' ? 'flex' : 'winkeyer');
+      setWkEngine(s.engine === 'icom' ? 'icom' : s.engine === 'flex' ? 'flex' : s.engine === 'serial' ? 'serial' : 'winkeyer');
     } catch { /* keyer not configured */ }
   }, []);