fix(frontend): harden auth routing and callback error handling

frontend/src/pages/AuthCallbackPage.vue

@@ -32,7 +32,17 @@ const error = ref('')
 onMounted(async () => {
   try {
     const code = route.query.code
-    const state = route.query.state
+    const oauthError = route.query.error
+    const oauthErrorDesc = route.query.error_description
+
+    if (oauthError) {
+      const reason = typeof oauthErrorDesc === 'string' && oauthErrorDesc
+        ? oauthErrorDesc
+        : String(oauthError)
+      error.value = `登入失敗：${reason}`
+      setTimeout(() => router.push('/login'), 3000)
+      return
+    }
 
     if (!code) {
       error.value = '缺少驗證代碼，登入失敗'