Show ip address in main menu

[Alex37894]

Fixes: #15927

Add compact, short information about your PR for easier understanding:

Goal of the PR: Display the player's local IP address in the top-left corner of the main menu to facilitate LAN multiplayer.

How does the PR work ? :

1. It adds a new C++ backend function l_get_local_ip in src/script/lua_api/l_mainmenu.cpp using getifaddrs to fetch local IPv4 addresses.
2. It safely filters out loopback addresses (the 127.0.0.0/8 subnet) using bitwise masking (INADDR_LOOPBACK & IN_CLASSA_NET).
3. It exposes this function to the Main Menu Lua environment as core.get_local_ip().
4. In builtin/mainmenu/init.lua, it overrides core.set_topleft_text to append the retrieved IP address right below the engine version text.

Does it resolve any reported issue ?:
Yes, resolves #15927 (#15927).

Does this relate to a goal in the roadmap?:
Not directly, but it improves the general User Experience (UX) for multiplayer networking.

If this PR doesn't fix a bug, why is it necessary? What use cases does it solve ?
As requested in the ticket, it simplifies hosting local network games. Players no longer need to open a terminal or dig through operating system settings to find their local IP address; it's now directly visible on the home screen.

To do

This PR is a Ready for Review.

How to test

1 - Compile the Luanti engine on a Linux or macOS machine.

2 - Launch the game.

3 - On the main menu screen, look at the top-left corner (under the Luanti version and debug text).

4 - You should see IP Locale: [Your Local Network IP] (e.g., 192.168.1.50).

5 - Ensure it does not display 127.0.0.1.

[rubenwardy]

This is wayy too prominent. If you think about when it's needed - someone joining your game - then just the pause menu should have this imo

[Alex37894]

C'est beaucoup trop visible. Si on réfléchit aux situations où c'est nécessaire (par exemple, lorsqu'un joueur rejoint votre partie), alors, à mon avis, seul le menu pause devrait afficher cette information.

Maybeit should only be displayed in the menu where you create/join a world ?

[siliconsniffer]

I don't really mind the IP address in the main menu but it should definitely be shown in the pause menu.

Since it's now enabled by default, does this work with IPv6 only? It should probably show a non-link-local one (ULA?).