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).

frontend/wailsjs/go/models.ts

@@ -1930,6 +1930,7 @@ export namespace main {
 	    kenwood_host: string;
 	    kenwood_port: string;
 	    kenwood_baud: number;
+	    kenwood_data_mode: string;
 	    yaesu_low_lines: boolean;
 	    kenwood_low_lines: boolean;
 	    icom_port: string;
@@ -1975,6 +1976,7 @@ export namespace main {
 	        this.kenwood_host = source["kenwood_host"];
 	        this.kenwood_port = source["kenwood_port"];
 	        this.kenwood_baud = source["kenwood_baud"];
+	        this.kenwood_data_mode = source["kenwood_data_mode"];
 	        this.yaesu_low_lines = source["yaesu_low_lines"];
 	        this.kenwood_low_lines = source["kenwood_low_lines"];
 	        this.icom_port = source["icom_port"];