refactor(cat): one DTR/RTS setting per backend, none shared

The 0.22.7/0.22.8 breakage came from a Xiegu-reported fault being fixed
inside the Yaesu and Kenwood backends. The lesson is structural: nothing
a backend does may be steered by another backend's report or another
backend's setting.

The lower-lines checkbox introduced for 0.22.9 was still one shared key
applied to both Yaesu and Kenwood — the same reflex in miniature. It is
now cat.yaesu.low_dtr_rts and cat.kenwood.low_dtr_rts, each checkbox in
its backend's own settings block, each wired only to its backend. The
shared key never shipped, so nothing needs migrating.

frontend/wailsjs/go/models.ts

@@ -1924,7 +1924,8 @@ export namespace main {
 	    kenwood_host: string;
 	    kenwood_port: string;
 	    kenwood_baud: number;
-	    lower_lines: boolean;
+	    yaesu_low_lines: boolean;
+	    kenwood_low_lines: boolean;
 	    icom_port: string;
 	    icom_baud: number;
 	    icom_addr: number;
@@ -1965,7 +1966,8 @@ export namespace main {
 	        this.kenwood_host = source["kenwood_host"];
 	        this.kenwood_port = source["kenwood_port"];
 	        this.kenwood_baud = source["kenwood_baud"];
-	        this.lower_lines = source["lower_lines"];
+	        this.yaesu_low_lines = source["yaesu_low_lines"];
+	        this.kenwood_low_lines = source["kenwood_low_lines"];
 	        this.icom_port = source["icom_port"];
 	        this.icom_baud = source["icom_baud"];
 	        this.icom_addr = source["icom_addr"];