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/util/time.rs

@@ -4,11 +4,14 @@ use std::time::Duration;
 use chrono::{DateTime, Utc};
 use tracing::{debug, warn, error};
 
+#[allow(dead_code)]
 const TIME_SYNC_URL: &str = "https://core.telegram.org/getProxySecret";
+#[allow(dead_code)]
 const MAX_TIME_SKEW_SECS: i64 = 30;
 
 /// Time sync result
 #[derive(Debug, Clone)]
+#[allow(dead_code)]
 pub struct TimeSyncResult {
     pub server_time: DateTime<Utc>,
     pub local_time: DateTime<Utc>,
@@ -17,6 +20,7 @@ pub struct TimeSyncResult {
 }
 
 /// Check time synchronization with Telegram servers
+#[allow(dead_code)]
 pub async fn check_time_sync() -> Option<TimeSyncResult> {
     let client = reqwest::Client::builder()
         .timeout(Duration::from_secs(10))
@@ -60,6 +64,7 @@ pub async fn check_time_sync() -> Option<TimeSyncResult> {
 }
 
 /// Background time sync task
+#[allow(dead_code)]
 pub async fn time_sync_task(check_interval: Duration) -> ! {
     loop {
         if let Some(result) = check_time_sync().await {