feat: Denkovi USB 4/8 (selectable) + generic USB-serial relay board (CH340/LCUS)

- Denkovi (FT245 D2XX) now selectable as 4 or 8 relays (a 4-relay board just uses
  the low nibble); NewDenkovi takes a count, driven by the device's Channels.
- New 'usbrelay' backend: cheap CH340/LCUS USB-serial boards over a COM port
  using the common A0 protocol ([0xA0][relay][state][checksum]); write-only, so
  Status is a shadow. Channel count configurable (1/2/4/8/16).
- StationDevice gains Channels; deviceRelayCount() drives label/relay counts for
  the variable-size types. Device editor: channel selector + COM-port picker
  (usbrelay) / FTDI-serial (denkovi).

Both untested on hardware; the A0 command set / Denkovi bit order may need a tweak
after a live test.

frontend/wailsjs/go/models.ts

@@ -2600,6 +2600,7 @@ export namespace main {
 	    host: string;
 	    user?: string;
 	    pass?: string;
+	    channels?: number;
 	    labels: string[];
 	
 	    static createFrom(source: any = {}) {
@@ -2614,6 +2615,7 @@ export namespace main {
 	        this.host = source["host"];
 	        this.user = source["user"];
 	        this.pass = source["pass"];
+	        this.channels = source["channels"];
 	        this.labels = source["labels"];
 	    }
 	}