fix: friendly Chinese error messages on login/register pages

Map backend English error details to user-facing Chinese text on all
3 frontends (uni-app, user-frontend, admin-frontend). Previously raw
English like 'Invalid credentials' was shown directly to users.

admin-frontend/src/views/Landing.vue

@@ -90,7 +90,8 @@ async function submit() {
     localStorage.setItem('admin_user', JSON.stringify(res.user || {}))
     ElMessage.success('登录成功')
   } catch (e) {
-    error.value = e?.detail || '登录失败'
+    const map = { 'Invalid credentials': '用户名或密码错误' }
+    error.value = map[e?.detail] || e?.detail || '登录失败'
   } finally {
     loading.value = false
   }

admin-frontend/src/views/Login.vue

@@ -43,7 +43,8 @@ async function submit() {
     const redirect = route.query.redirect || '/dashboard'
     router.push(redirect)
   } catch (e) {
-    error.value = e?.detail || '登录失败'
+    const map = { 'Invalid credentials': '用户名或密码错误' }
+    error.value = map[e?.detail] || e?.detail || '登录失败'
   } finally {
     loading.value = false
   }