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).

internal/winkeyer/winkeyer.go

@@ -58,6 +58,7 @@ type Config struct {
 	//                 apply; the paddle/sidetone/ratio fields are WinKeyer-only.
 	Type      string `json:"type"`
 	CWKeyLine string `json:"cw_key_line"` // serial: "dtr" (CW on DTR, PTT on RTS — default) | "rts"
+	CWInvert  bool   `json:"cw_invert"`   // serial: invert line polarity (active-LOW) for both key and PTT
 }
 
 func (c Config) normalised() Config {