Do not trust `v-model` in login input, for password manager's autofill

Changes

• Make login successful after password manager (specifically Keepass Tusk) autofills.
  • Previously it sent {username: undefined, password: undefined} to the backend, even if it's filled.

The v-model is not updated after the autofill somehow. Instead of adding more listener like onchange, this tries to don't rely on v-model and get username and password from $refs.