feat: add field-level error metadata to register endpoint responses

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/api/auth/register.post.ts

@@ -12,13 +12,14 @@ export default defineWrappedResponseHandler(async (event) => {
 
   const parsed = registerSchema.safeParse(body)
   if (!parsed.success) {
-    return R.error(parsed.error.issues[0]?.message || '表单验证失败', null)
+    const field = parsed.error.issues[0]?.path[0]?.toString() ?? undefined
+    return R.error(parsed.error.issues[0]?.message || '表单验证失败', { field })
   }
 
   const { username, password, captchaToken, captchaText } = parsed.data
 
   if (!verifyCaptcha(captchaToken, captchaText)) {
-    return R.error('验证码错误或已过期', null)
+    return R.error('验证码错误或已过期', { field: 'captchaText' })
   }
 
   const hashedPassword = await hash(password, 10)
@@ -38,7 +39,7 @@ export default defineWrappedResponseHandler(async (event) => {
   } catch (err: any) {
     const msg = String(err?.message ?? '')
     if (msg.toLowerCase().includes('unique') || msg.includes('SQLITE_CONSTRAINT')) {
-      return R.error('用户名已存在', null)
+      return R.error('用户名已存在', { field: 'username' })
     }
     logger.error('Failed to insert user', msg)
     return R.error('注册失败，请稍后重试', null)