Fix subdirectory multisite wp-login.php routing and align $wpSiteUrl default

[KreigD]

Summary

This PR fixes an edge case in Herd/Valet subdirectory multisite routing where subsite login URLs can be routed as front-end page requests instead of the core login script.

Problem

In subdirectory multisite setups, requests like:

• https://example.test/site-a/wp-admin/
• https://example.test/site-a/wp-login.php

should resolve to core admin/login behavior for that subsite.
The driver already normalizes wp-admin, wp-content, and wp-includes, but not wp-login.php inside frontControllerPath().

In Herd/Valet, .php handling goes through front controller resolution logic, so missing wp-login.php normalization can cause:

• self-redirect/login loop behavior on subsite login URLs
• reauth redirects that never reach a real login form for some flows

Changes

1. Normalize wp-login.php in frontControllerPath()
   Include wp-login.php in the same URI normalization branch used for wp-admin, wp-content, and wp-includes.

2. Align default $wpSiteUrl with README guidance
   Change default from "/" to "" for vanilla installs (as documented in README), reducing path concatenation ambiguity.

Why this is safe

• wp-login.php is a core path in the same family as other normalized WordPress core routes.
• Existing behavior for wp-admin/network remains unchanged.
• This improves parity between documented defaults and runtime defaults.

Reference

Observed and reproduced on Laravel Herd with WordPress multisite subdirectory install.