fix(web): complete pluginized agent onboarding flows

web/src/app/infra/http/backendUrl.ts

@@ -0,0 +1,24 @@
+export function resolveBackendBaseUrl(
+  configuredBaseUrl: string | undefined,
+  origin: string,
+): string {
+  const normalizedOrigin = origin.replace(/\/+$/, '');
+  const configured = configuredBaseUrl?.trim() ?? '';
+
+  if (!configured || configured === '/') {
+    return normalizedOrigin;
+  }
+
+  if (/^https?:\/\//i.test(configured)) {
+    return configured.replace(/\/+$/, '');
+  }
+
+  return `${normalizedOrigin}/${configured.replace(/^\/+|\/+$/g, '')}`;
+}
+
+export function getBackendBaseUrl(): string {
+  return resolveBackendBaseUrl(
+    import.meta.env.VITE_API_BASE_URL,
+    window.location.origin,
+  );
+}