Bug: Routes return 404 on Windows

[rogerio-gmm]

Description:
Routes are not being registered correctly on Windows, causing all requests to return 404 Not Found errors.

Environment:

• OS: Windows 10/11
• Node.js: 24.8.0
• the-fake-backend: 3.12.0

Steps to Reproduce:

1. Create a server with routes on Windows
2. Make a request to any route
3. Receive 404 instead of expected response

Expected Behavior:
Routes should work on Windows like they do on macOS

Root Cause:
The code uses path.join() which uses backslashes on Windows, but Express routes need forward slashes.

Proposed Solution:
Use path.posix.join() instead of path.join() for route paths.

I have a fix ready #137 if this is acceptable.