Update to use /dashboard subpath

[nikolaischunk]

Description

currently, the application is hosted on dashboard.birthdayy.xyz.
In the Future we want to use https://birthdayy.xyz/dashboard as path.

(Maybe don't use /dashboard on localhost)

• use the redirect option to customize the redirect URL

https://github.com/BirthdayyBot/dashboard/blob/501ca2ce1b13f7d024f0e427cf7035fa417f0ac7/src/lib/options/auth.ts

• add basePath attribute here

https://github.com/BirthdayyBot/dashboard/blob/02d26a31b6221269e856675e1cf2685930e42381/src/lib/provider/auth.tsx

• Add NEXTAUTH_URL and NEXTAUTH_BASE_URL properties to ENV Variable -> /dashboard/api/auth

• configure basePath in the next.config.ts to /dashboard

• https://github.com/BirthdayyBot/dashboard/blob/8c8cc8a61352f63a8f02c50b29b3717623b90cd7/next.config.js