fix(admin): show login API error toast on 401

Anonymous auth 401s skip the global message handler, so wrong
credentials left the login form silent; surface the server detail.

Co-authored-by: Cursor <cursoragent@cursor.com>

admin/src/views/auth/login/index.vue

@@ -161,7 +161,7 @@
       })
 
       if (!result?.token) {
-        throw new Error('Login failed - no token received')
+        throw new Error('登录失败，未返回令牌')
       }
 
       const permissions = result.permissions || []
@@ -181,8 +181,12 @@
       const redirect = route.query.redirect as string
       router.push(redirect || '/')
     } catch (error) {
-      if (!(error instanceof HttpError)) {
+      // 匿名登录接口的 401 不会走全局 showError，需在此提示服务端 detail。
+      if (error instanceof HttpError) {
+        ElMessage.error(error.message)
+      } else {
         console.error('[Login] Unexpected error:', error)
+        ElMessage.error(error instanceof Error ? error.message : '登录失败，请稍后重试')
       }
       await loadCaptcha()
     } finally {