refactor: de-duplicate channel runtime and payload helpers

src/shared/gateway-bind-url.ts

@@ -0,0 +1,45 @@
+export type GatewayBindUrlResult =
+  | {
+      url: string;
+      source: "gateway.bind=custom" | "gateway.bind=tailnet" | "gateway.bind=lan";
+    }
+  | {
+      error: string;
+    }
+  | null;
+
+export function resolveGatewayBindUrl(params: {
+  bind?: string;
+  customBindHost?: string;
+  scheme: "ws" | "wss";
+  port: number;
+  pickTailnetHost: () => string | null;
+  pickLanHost: () => string | null;
+}): GatewayBindUrlResult {
+  const bind = params.bind ?? "loopback";
+  if (bind === "custom") {
+    const host = params.customBindHost?.trim();
+    if (host) {
+      return { url: `${params.scheme}://${host}:${params.port}`, source: "gateway.bind=custom" };
+    }
+    return { error: "gateway.bind=custom requires gateway.customBindHost." };
+  }
+
+  if (bind === "tailnet") {
+    const host = params.pickTailnetHost();
+    if (host) {
+      return { url: `${params.scheme}://${host}:${params.port}`, source: "gateway.bind=tailnet" };
+    }
+    return { error: "gateway.bind=tailnet set, but no tailnet IP was found." };
+  }
+
+  if (bind === "lan") {
+    const host = params.pickLanHost();
+    if (host) {
+      return { url: `${params.scheme}://${host}:${params.port}`, source: "gateway.bind=lan" };
+    }
+    return { error: "gateway.bind=lan set, but no private LAN IP was found." };
+  }
+
+  return null;
+}