fix: resolve all 40 clippy warnings (dead_code, unused_imports, lints)

- Add #[allow(unused_imports)] on pub re-exports used only by lib/tests
- Add #[allow(dead_code)] on public API items unused in binary target
- Fix collapsible_if, redundant_closure, unnecessary_map_or in main.rs
- Prefix unused test variables with underscore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tdlib/users.rs

@@ -213,6 +213,7 @@ impl UserCache {
     /// # Returns
     ///
     /// Имя пользователя (first_name + last_name) или "User {id}" если не найден.
+    #[allow(dead_code)]
     pub async fn get_user_name(&self, user_id: UserId) -> String {
         // Сначала пытаемся получить из кэша
         if let Some(name) = self.user_names.peek(&user_id) {