feat: Add fallback placeholder for the login password field

Proposed changes

Add a fallback translation to the password placeholder in LoginForm.tsx

Currently, the login password field uses:
placeholder={passwordPlaceholder}

If Accounts_PasswordPlaceholder has not been configured by the administrator, passwordPlaceholder can be empty, causing the password input to render without placeholder text.

As you can clearly see here the placeholder is not there in password field.
so its a good upgrade for the UI to stay presentable

Steps to reproduce :

1. Open the Rocket.Chat login page.
2. Ensure Accounts_PasswordPlaceholder is not configured.
3. Observe the username/email input.
4. Observe the password input.
5. Compare the placeholder behavior of both fields.
6. The username/email field displays its fallback placeholder, while the password field has no placeholder text.

Before proof :

Current implementation in LoginForm.tsx:
placeholder={passwordPlaceholder}

When passwordPlaceholder is empty, the rendered password field has no placeholder.

Meanwhile, the username/email field uses:

placeholder={
usernameOrEmailPlaceholder ||
t('registration.component.form.emailPlaceholder')
}

and therefore still displays a fallback.

After proof

Expected implementation:

placeholder={
passwordPlaceholder ||
t('registration.component.form.passwordPlaceholder')
}

Expected result:

Username / Email
[ example@example.com ]

Password
[ Enter your password ]

i would like to work on this and make this changes

Start in LoginForm.tsx and compare the password field with the username/email field's existing fallback placeholder behavior. Reproduce the login page with Accounts_PasswordPlaceholder unset, then verify the password input displays the fallback text while preserving the configured placeholder when one exists.