feat(web): warn on default-password accounts

Track built-in admin and user accounts that still use their
seeded password so the web UI can prompt operators to
rotate credentials after deployment.

- Persist must-change-password state for seeded accounts.
- Clear the reminder after password changes and validate
  empty-password updates.
- Keep the migration and auth API behavior explicit.

easytier-web/frontend/src/components/Login.vue

@@ -7,6 +7,7 @@ import { I18nUtils } from 'easytier-frontend-lib';
 import { getInitialApiHost, cleanAndLoadApiHosts, saveApiHost } from "../modules/api-host"
 import { useI18n } from 'vue-i18n'
 import ApiClient, { Credential, RegisterData } from '../modules/api';
+import { setMustChangePasswordFlag } from '../modules/auth-status';
 
 const { t } = useI18n()
 
@@ -33,6 +34,7 @@ const onSubmit = async () => {
     let ret = await api.value?.login(credential);
     if (ret.success) {
         localStorage.setItem('apiHost', btoa(apiHost.value));
+        setMustChangePasswordFlag(Boolean(ret.mustChangePassword));
         router.push({
             name: 'dashboard',
             params: { apiHost: btoa(apiHost.value) },