Support configurable login form selectors for non-standard label names

[onchul]

Problem

The auth action finds login fields using getByLabel(/user ?name/i) and getByLabel(/password/i). This misses common login forms:

• Email-based login: "Email", "E-Mail" don't match /user ?name/i
• Non-English apps: "Benutzername", "Mot de passe" don't match
• No visible labels: Forms using only placeholder attributes

When no match is found, the login form is not detected and the scanner proceeds unauthenticated.

Suggested fix

1. Fall back to input types when label matching fails: input[type="password"] for the password field, and the closest preceding input[type="text"] or input[type="email"] for the username field.
2. Make selectors configurable via optional inputs like username_selector and password_selector, so users can specify their own labels, placeholders, or CSS selectors for non-standard forms.