feat(cat): per-rig Kenwood/Elecraft data-mode choice (USB / MD6 / keep)

No single Kenwood mode digit fits every rig for a data mode: a K3/K4 wants
DATA (MD6), a TS-590SG/TS-990S data mode is a USB modifier set on the rig,
and MD6 on a plain Kenwood is FSK/RTTY. So the operator now chooses in
Settings → CAT: USB (default), DATA (MD6), or leave the rig's mode unchanged.

Only soundcard/data modes (FT8, PSK, JT…) follow this; RTTY/FSK stays its own
native mode, and voice/CW are untouched (isKenwoodDataMode + test).

internal/cat/kenwood_test.go

@@ -111,6 +111,23 @@ func TestKenwoodModeDigit(t *testing.T) {
 	}
 }
 
+// The DATA-mode preference only applies to soundcard/data modes, never to voice,
+// CW or native RTTY — getting this wrong would hijack an SSB or CW mode change.
+func TestIsKenwoodDataMode(t *testing.T) {
+	data := []string{"FT8", "FT4", "PSK31", "JT65", "DATA", "DIGITAL", "OLIVIA"}
+	notData := []string{"", "LSB", "USB", "SSB", "CW", "CW-R", "FM", "AM", "RTTY", "FSK", "RTTY-R"}
+	for _, m := range data {
+		if !isKenwoodDataMode(m) {
+			t.Errorf("isKenwoodDataMode(%q) = false, want true", m)
+		}
+	}
+	for _, m := range notData {
+		if isKenwoodDataMode(m) {
+			t.Errorf("isKenwoodDataMode(%q) = true, want false", m)
+		}
+	}
+}
+
 // What the log records for each mode digit the rig reports.
 func TestKenwoodModeToADIF(t *testing.T) {
 	cases := []struct {