feat(relay): accept a full URL host so relays work behind an HTTPS proxy

Network relay boards (WebSwitch/KMTronic/Dingtian) were always reached at
http://<host>, so a board on the LAN behind a reverse proxy — the common remote
setup where the operator's 80/443 already go to Nginx Proxy Manager — couldn't be
reached from outside. relayBase() now keeps a scheme the operator supplies
(https://relay.example.com, optional sub-path) and only defaults to http:// for a
bare host/host:port. UI hint + test added.

internal/relaydev/relaydev_test.go

@@ -172,3 +172,19 @@ func TestDingtianSetRefusalIsAnError(t *testing.T) {
 		t.Fatal("a refused command answered HTTP 200 and was reported as success")
 	}
 }
+
+func TestRelayBase(t *testing.T) {
+	cases := map[string]string{
+		"192.168.1.5":                  "http://192.168.1.5",
+		"192.168.1.5:8080":             "http://192.168.1.5:8080",
+		"https://relay.example.com":    "https://relay.example.com",
+		"https://relay.example.com/":   "https://relay.example.com",
+		"http://relay.example.com/sw/": "http://relay.example.com/sw",
+		"  relay.local  ":              "http://relay.local",
+	}
+	for in, want := range cases {
+		if got := relayBase(in); got != want {
+			t.Errorf("relayBase(%q) = %q, want %q", in, got, want)
+		}
+	}
+}