feat: serial CW keyer — add invert (active-LOW) polarity option

Some CW interfaces key on the opposite line level (transmit at rest / key
backwards). New "Invert keying (active-LOW)" toggle flips the asserted level for
both the CW and PTT lines, and the idle state drives both lines to their
inactive level accordingly so an active-low rig isn't left keyed at rest.
Default off = N1MM convention (line HIGH = active).

frontend/src/components/SettingsModal.tsx

@@ -1074,13 +1074,13 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
     port: string; baud: number; wpm: number; weight: number;
     lead_in_ms: number; tail_ms: number; ratio: number; farnsworth: number;
     sidetone_hz: number; mode: string; swap: boolean; autospace: boolean;
-    use_ptt: boolean; serial_echo: boolean; cw_key_line: string; macros: WKMac[];
+    use_ptt: boolean; serial_echo: boolean; cw_key_line: string; cw_invert: boolean; macros: WKMac[];
   };
   const [wk, setWk] = useState<WKSettings>({
     enabled: false, engine: 'winkeyer', esc_clears_call: true,
     port: '', baud: 1200, wpm: 25, weight: 50, lead_in_ms: 10,
     tail_ms: 50, ratio: 50, farnsworth: 0, sidetone_hz: 600, mode: 'iambic_b',
-    swap: false, autospace: true, use_ptt: false, serial_echo: true, cw_key_line: 'dtr', macros: [],
+    swap: false, autospace: true, use_ptt: false, serial_echo: true, cw_key_line: 'dtr', cw_invert: false, macros: [],
   });
   const [wkPorts, setWkPorts] = useState<string[]>([]);
   const setWkField = (patch: Partial<WKSettings>) => setWk((s) => ({ ...s, ...patch }));
@@ -3131,6 +3131,10 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                   <Checkbox checked={wk.use_ptt} onCheckedChange={(c) => setWkField({ use_ptt: !!c })} /> Key PTT line
                 </label>
               </div>
+              <label className="flex items-center gap-2 text-sm cursor-pointer">
+                <Checkbox checked={wk.cw_invert} onCheckedChange={(c) => setWkField({ cw_invert: !!c })} />
+                Invert keying (active-LOW) — tick this only if the rig transmits at rest / keys backwards
+              </label>
             </>
           ) : (
             <>