fix(frontend): remove stray reference to deleted upstreamType ref

resetForm 中残留 upstreamType.value = 'sub2api' 引用,但变量本身已在
commit 8d0ef3d2 (antigravity 瘦身) 删除,导致 vue-tsc 编译失败:

  CreateAccountModal.vue(4249,3): error TS2304:
  Cannot find name 'upstreamType'.

之前 merge 时误以为这是 stray 改动并 revert 了删除。实际上 8d0ef3d2
删除了 upstreamType ref 声明和所有使用位点,但漏删了 resetForm 中的
赋值,需要补上。

frontend/src/components/account/CreateAccountModal.vue

@@ -4246,7 +4246,6 @@ const resetForm = () => {
   customBaseUrl.value = ''
   allowOverages.value = false
   antigravityAccountType.value = 'oauth'
-  upstreamType.value = 'sub2api'
   upstreamBaseUrl.value = ''
   upstreamApiKey.value = ''
   vertexServiceAccountJson.value = ''