Block UN-renderer

Server-side opt-in for the see-through feature, per Modrinth Content Rules section 3 (seeing through opaque blocks requires a server-side opt-in).

• Singleplayer / your own world: works exactly as before.
• Multiplayer: disabled by default. A server enables it by installing the mod on the server side and setting allowSeeThrough=true in config/block-unrenderer-server.json; the server then sends an opt-in handshake to clients on join.
• On vanilla servers (or any that have not opted in) the feature stays disabled.
• ModMenu is now optional. The mod can load on dedicated servers only to send the opt-in handshake, and never changes world or block data.