[BUG] Change cookie name #155

Describe the bug
We really need the ability to change cookie name, and that's why

To Reproduce
Steps to reproduce the behavior:

1. Create tinyauth.d1.com and so on, everything fine
2. Create another instance on another machine tinyauth.s1.d1.com and it looks fine but
3. Now we have (in browser) 2 cookies with arbitrary name tinyauth and distinguished Domain fields (.d1.com and .s1.d1.com)
4. Root (d1.com) works fine, but s1.d1.com can't login, after user/pass or oidc just redirects to login page and

tinyauth           | 2025-05-19T22:11:44Z ERR Failed to get session error="securecookie: the value is not valid"
tinyauth           | 2025-05-19T22:11:44Z ERR Failed to get session error="securecookie: the value is not valid"
tinyauth           | 2025-05-19T22:11:44Z ERR Failed to get session cookie error="securecookie: the value is not valid"

Expected behavior
Just works

Additional context
Manual removing cookies helps till login to both d1.com and s1.d1.com, then only first one works. I'm really not sure but suppose that problem should gone if cookie name will differ.