Extract backend API utilities into src/backendApi.js

src/backendApi.js

@@ -0,0 +1,34 @@
+export function isAbortError(error) {
+  return error?.name === 'AbortError'
+}
+
+export function getErrorText(error) {
+  if (error instanceof Error && error.message) return error.message
+  return String(error)
+}
+
+export async function readBackendErrorText(response) {
+  const bodyText = await response.text().catch(() => '')
+  if (bodyText) {
+    try {
+      const data = JSON.parse(bodyText)
+      if (typeof data?.detail === 'string' && data.detail.trim()) {
+        return data.detail.trim()
+      }
+      if (typeof data?.message === 'string' && data.message.trim()) {
+        return data.message.trim()
+      }
+    } catch {}
+    return bodyText.trim()
+  }
+  return `HTTP ${response.status}`
+}
+
+export async function expectBackendJson(response) {
+  const data = await response.json().catch(() => null)
+  if (response.ok) return data
+  const detail = typeof data?.detail === 'string'
+    ? data.detail
+    : (typeof data?.message === 'string' ? data.message : '')
+  throw new Error(detail || `HTTP ${response.status}`)
+}