fix(setup): align install validation and expose backend errors

Make setup password requirements consistent with backend rules and show API-provided error messages so install failures are actionable. Trim admin email before validation to avoid false invalid-email rejections from surrounding whitespace.

frontend/src/i18n/locales/zh.ts

@@ -218,7 +218,7 @@ export default {
       email: '邮箱',
       password: '密码',
       confirmPassword: '确认密码',
-      passwordPlaceholder: '至少 6 个字符',
+      passwordPlaceholder: '至少 8 个字符',
       confirmPasswordPlaceholder: '确认密码',
       passwordMismatch: '密码不匹配'
     },