fix: eliminate all compiler warnings

- Remove unused imports across multiple modules
- Add #![allow(dead_code)] for public API items preserved for future use
- Add #![allow(deprecated)] for rand::Rng::gen_range usage
- Add #![allow(unused_assignments)] in main.rs
- Add #![allow(unreachable_code)] in network/stun.rs
- Prefix unused variables with underscore (_ip_tracker, _prefer_ipv6)
- Fix unused_must_use warning in tls_front/cache.rs

This ensures clean compilation without warnings while preserving
public API items that may be used in the future.

src/proxy/mod.rs

@@ -8,6 +8,9 @@ pub mod middle_relay;
 pub mod relay;
 
 pub use client::ClientHandler;
+#[allow(unused_imports)]
 pub use handshake::*;
+#[allow(unused_imports)]
 pub use masking::*;
+#[allow(unused_imports)]
 pub use relay::*;