feat(relays): generic HTTP relay board, and point band switching at it

An operator was about to build band→relay mapping into the custom URL rows.
It already exists, and better: relay auto-control has a per-relay "band" rule
driving webswitch, KMTronic, Dingtian, Denkovi and USB boards. It holds state
so a relay is only commanded when its wanted position changed, reads the
boards live before the first apply so it does not re-command one already in
place, and carries per-relay labels. A URL fired on a band change has none of
that, and would have been a second definition of "which antenna on which band"
— the drift that cost us a week on counties.

So the gap was only the hardware: a hand-made switch is none of the five named
types. It is now the sixth, "HTTP relay": an ON URL and an OFF URL, either as
one pattern with {relay} substituted, or one pair per relay. The per-relay
form is the reason it exists — these boxes often have URLs with nothing in
common between channels (…/FF0101 and …/FF0201), which no pattern can express.

Status is remembered rather than read: most have no endpoint worth trusting.
The cost is stated in the code and the panel — after a restart every relay is
re-commanded once, which is harmless on a board with no memory and far better
than assuming an antenna is already selected.

And the Connections panel now says so, exactly where the wrong choice is made:
picking a band change with a URL transport shows a note pointing at Station
Control. The URL transport stays — a lookup pushed to a webhook or a QSO to a
dashboard is an event, not a state, and Station Control has no place for it.

app.go

@@ -14635,26 +14635,39 @@ func boolStr(b bool) string {
 // StationDevice is one configured relay board for the Station Control tab.
 type StationDevice struct {
 	ID       string   `json:"id"`
-	Type     string   `json:"type"` // "webswitch" | "kmtronic" | "dingtian" | "denkovi" | "usbrelay"
+	Type     string   `json:"type"` // "webswitch" | "kmtronic" | "dingtian" | "denkovi" | "usbrelay" | "httpgen"
 	Name     string   `json:"name"`
 	Host     string   `json:"host"`               // IP for network boards; FTDI serial for denkovi; COM port for usbrelay
 	User     string   `json:"user,omitempty"`     // KMTronic HTTP auth; Dingtian HTTP session ID (blank = none)
 	Pass     string   `json:"pass,omitempty"`     // KMTronic HTTP auth; Dingtian relay password (0–9999)
 	Channels int      `json:"channels,omitempty"` // usbrelay/denkovi/dingtian: number of relays
 	Labels   []string `json:"labels"`             // per-relay label (index 0 = relay 1)
+
+	// Generic HTTP board only. Per-relay URLs win over the patterns, which is
+	// what lets a hand-made switch with nothing in common between its channels
+	// be described at all.
+	OnURLs  []string `json:"on_urls,omitempty"` // index 0 = relay 1
+	OffURLs []string `json:"off_urls,omitempty"`
+	OnPat   string   `json:"on_pattern,omitempty"` // fallback, {relay} substituted
+	OffPat  string   `json:"off_pattern,omitempty"`
 }
 
 // deviceRelayCount is the relay count for a configured device — fixed by type,
 // except the Denkovi (4 or 8) and the generic USB-serial board, whose channel
 // count the user picks.
 func deviceRelayCount(d StationDevice) int {
-	if d.Type == "usbrelay" || d.Type == "denkovi" || d.Type == "dingtian" {
+	if d.Type == "usbrelay" || d.Type == "denkovi" || d.Type == "dingtian" || d.Type == "httpgen" {
 		if d.Channels >= 1 {
 			return d.Channels
 		}
 		if d.Type == "dingtian" {
 			return 2 // the smallest board; the real count comes from its status reply
 		}
+		if d.Type == "httpgen" {
+			// A hand-made switch is usually two to four ways, and its URLs are
+			// typed one per relay — eight empty rows would be eight rows to read.
+			return 4
+		}
 		return 8
 	}
 	return relayCountFor(d.Type)
@@ -14788,7 +14801,7 @@ func (a *App) SaveStationDevices(devs []StationDevice) error {
 	}
 	for i := range devs {
 		switch devs[i].Type {
-		case "kmtronic", "dingtian", "denkovi", "usbrelay", "webswitch":
+		case "kmtronic", "dingtian", "denkovi", "usbrelay", "webswitch", "httpgen":
 			// known type — keep
 		default:
 			devs[i].Type = "webswitch"