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.

从 auth action 入手，其中 getByLabel(/user ?name/i) 和 getByLabel(/password/i) 会定位登录字段，然后检查可选输入是如何定义的。支持可配置的 username_selector 和 password_selector 值以及提议的 input-type 回退方案，并验证能够检测到非标准表单或没有标签的表单，并在已认证状态下对其进行扫描。

由索引模型根据 Issue 内容生成。