chore: remove dead code and unnecessary allow(dead_code) attributes

Cleaned up warnings by removing unused code:
- Removed unused format_timestamp() function from utils.rs
- Removed unused len() method from LruCache
- Removed unused date field from ForwardInfo struct
- Removed unnecessary #[allow(dead_code)] attributes from:
  * AuthState enum (actually used)
  * ChatInfo struct (actually used)
  * TdClient impl block (actually used)

This reduces code noise and makes real warnings more visible.

Changes:
- 20 lines removed
- 1 line added
- 6 files changed

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/tdlib/client.rs

@@ -59,7 +59,6 @@ pub struct TdClient {
     pub network_state: NetworkState,
 }
 
-#[allow(dead_code)]
 impl TdClient {
     /// Creates a new TDLib client instance.
     ///
@@ -1102,7 +1101,7 @@ impl TdClient {
     fn extract_forward_info(&self, message: &TdMessage) -> Option<ForwardInfo> {
         message.forward_info.as_ref().map(|info| {
             let sender_name = self.get_origin_sender_name(&info.origin);
-            ForwardInfo { sender_name, date: info.date }
+            ForwardInfo { sender_name }
         })
     }