fix(frontend): upstream account edit fields and mixed_scheduling on create

- EditAccountModal: add Base URL / API Key fields for upstream type
- EditAccountModal: initialize editBaseUrl from credentials on upstream account open
- EditAccountModal: save upstream credentials (base_url, api_key) on submit
- CreateAccountModal: pass mixed_scheduling extra when creating upstream account

frontend/src/components/account/CreateAccountModal.vue

@@ -2714,7 +2714,8 @@ const handleSubmit = async () => {
 
     submitting.value = true
     try {
-      await createAccountAndFinish(form.platform, 'upstream', credentials)
+      const extra = mixedScheduling.value ? { mixed_scheduling: true } : undefined
+      await createAccountAndFinish(form.platform, 'upstream', credentials, extra)
     } catch (error: any) {
       appStore.showError(error.response?.data?.detail || t('admin.accounts.failedToCreate'))
     } finally {