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/tls_front/cache.rs

@@ -19,6 +19,7 @@ pub struct TlsFrontCache {
     disk_path: PathBuf,
 }
 
+#[allow(dead_code)]
 impl TlsFrontCache {
     pub fn new(domains: &[String], default_len: usize, disk_path: impl AsRef<Path>) -> Self {
         let default_template = ParsedServerHello {
@@ -173,7 +174,7 @@ impl TlsFrontCache {
         tokio::spawn(async move {
             loop {
                 for domain in &domains {
-                    fetcher(domain.clone()).await;
+                    let _ = fetcher(domain.clone()).await;
                 }
                 sleep(interval).await;
             }