Support configurable login form selectors for non-standard label names

## 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.