feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

frontend/src/utils/apiError.ts

@@ -0,0 +1,71 @@
+/**
+ * Centralized API error message extraction
+ *
+ * The API client interceptor rejects with a plain object: { status, code, message, error }
+ * This utility extracts the user-facing message from any error shape.
+ */
+
+interface ApiErrorLike {
+  status?: number
+  code?: number | string
+  message?: string
+  error?: string
+  reason?: string
+  metadata?: Record<string, unknown>
+  response?: {
+    data?: {
+      detail?: string
+      message?: string
+      code?: number | string
+    }
+  }
+}
+
+/**
+ * Extract the error code from an API error object.
+ */
+export function extractApiErrorCode(err: unknown): string | undefined {
+  if (!err || typeof err !== 'object') return undefined
+  const e = err as ApiErrorLike
+  const code = e.code ?? e.reason ?? e.response?.data?.code
+  return code != null ? String(code) : undefined
+}
+
+/**
+ * Extract a displayable error message from an API error.
+ *
+ * @param err - The caught error (unknown type)
+ * @param fallback - Fallback message if none can be extracted (use t('common.error') or similar)
+ * @param i18nMap - Optional map of error codes to i18n translated strings
+ */
+export function extractApiErrorMessage(
+  err: unknown,
+  fallback = 'Unknown error',
+  i18nMap?: Record<string, string>,
+): string {
+  if (!err) return fallback
+
+  // Try i18n mapping by error code first
+  if (i18nMap) {
+    const code = extractApiErrorCode(err)
+    if (code && i18nMap[code]) return i18nMap[code]
+  }
+
+  // Plain object from API client interceptor (most common case)
+  if (typeof err === 'object' && err !== null) {
+    const e = err as ApiErrorLike
+    // Interceptor shape: { message, error }
+    if (e.message) return e.message
+    if (e.error) return e.error
+    // Legacy axios shape: { response.data.detail }
+    if (e.response?.data?.detail) return e.response.data.detail
+    if (e.response?.data?.message) return e.response.data.message
+  }
+
+  // Standard Error
+  if (err instanceof Error) return err.message
+
+  // Last resort
+  const str = String(err)
+  return str === '[object Object]' ? fallback : str
+}