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/src/components/SettingsModal.tsx

@@ -698,7 +698,7 @@ function RelayAutoPanel() {
         <div key={dev.id} className="rounded-md border border-border">
           <div className="px-3 py-1.5 border-b border-border bg-muted/40 text-xs font-semibold">{dev.name || dev.id}</div>
           <div className="divide-y divide-border/60">
-            {Array.from({ length: relayCountUI(dev.type) }, (_, i) => i + 1).map((relay) => {
+            {Array.from({ length: (dev.labels?.length || relayCountUI(dev.type)) }, (_, i) => i + 1).map((relay) => {
               const r = ruleFor(dev.id, relay);
               const label = (dev.labels?.[relay - 1] || '').trim() || `${t('relayauto.relay')} ${relay}`;
               return (