feat: Denkovi USB 8-relay board (FT245 D2XX bit-bang)

Third relay device alongside WebSwitch and KMTronic. Despite enumerating as a
COM port, this board is driven via FTDI D2XX synchronous bit-bang (one byte = 8
relays, bit 0 = relay 1), not serial ASCII — so we load ftd2xx.dll at runtime
(no CGO) and call FT_OpenEx/FT_SetBitMode/FT_Write, addressing the board by its
FTDI serial (e.g. DAE0006K), exactly like the vendor tool. Windows-only (stub
elsewhere); ListDenkoviDevices enumerates connected serials for the picker.

Wired into relaydev (Count/Status/Set), app.go (deviceDriver/relayCountFor/type
whitelist + ListDenkoviDevices binding), and the Station device editor (new type
+ FTDI-serial field with Detect). Untested on hardware; bit order / on-polarity
may need a flip after a live test.

frontend/wailsjs/go/main/App.js

@@ -1050,6 +1050,10 @@ export function ListCountries() {
   return window['go']['main']['App']['ListCountries']();
 }
 
+export function ListDenkoviDevices() {
+  return window['go']['main']['App']['ListDenkoviDevices']();
+}
+
 export function ListOperatingTree() {
   return window['go']['main']['App']['ListOperatingTree']();
 }