feat: admin authentication

[Thechi2000]

Allows admin authentication through the usage of an admin token. To simplify session management (and the rest of the project), migrates to use the App router, in a src directory.

Adds:

• A stub admin dashboard on /admin
• A login page on /login?type=admin. Can be later reused as /login?type=tequila for the users.
• Session management using JWTs in session.ts.
• Two functions for URL management: getUrl() and buildUrl().
  In order to access the full url anywhere in the project (on server-side), I added a middleware which includes the url in a next-url header. This header will only be present on the server, and can be accessed using the getUrl() function, which wraps around the native headers().

Session tokens are managed in session.ts, which contains server actions. Those are server-side functions that can be directly called from the frontend (which is so neat, I love it). It (un)sets a cookie named admin-session, containing a JWT payload (see https://jwt.io/introduction).

The layout in src/app/admin ensures that a user has a valid session before accessing any page beneath. If there is not, then the user is redirected to /login?type=admin, where they will have to enter the admin token.

All UIs are stub, meant to be improved.