feat(gateway): update runtime config guard for trusted-proxy

- Allow non-loopback bind with trusted-proxy auth mode
- Reject trusted-proxy + loopback combination (nonsensical)
- Require trustedProxies to be configured for trusted-proxy mode

Part of #1560

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/gateway/server-runtime-config.ts

@@ -74,6 +74,8 @@ export async function resolveGatewayRuntimeConfig(params: {
   const canvasHostEnabled =
     process.env.CLAWDBOT_SKIP_CANVAS_HOST !== "1" && params.cfg.canvasHost?.enabled !== false;
 
+  const trustedProxies = params.cfg.gateway?.trustedProxies ?? [];
+
   assertGatewayAuthConfigured(resolvedAuth);
   if (tailscaleMode === "funnel" && authMode !== "password") {
     throw new Error(
@@ -89,6 +91,20 @@ export async function resolveGatewayRuntimeConfig(params: {
     );
   }
 
+  // Trusted-proxy mode validations
+  if (authMode === "trusted-proxy") {
+    if (isLoopbackHost(bindHost)) {
+      throw new Error(
+        "gateway auth mode=trusted-proxy makes no sense with bind=loopback; use bind=lan or bind=custom with gateway.trustedProxies configured",
+      );
+    }
+    if (trustedProxies.length === 0) {
+      throw new Error(
+        "gateway auth mode=trusted-proxy requires gateway.trustedProxies to be configured with at least one proxy IP",
+      );
+    }
+  }
+
   return {
     bindHost,
     controlUiEnabled,