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>

tests/navigation.rs

@@ -74,7 +74,7 @@ async fn test_enter_opens_chat() {
 #[tokio::test]
 async fn test_esc_closes_chat() {
     // Состояние: открыт чат 123
-    let selected_chat_id = Some(123);
+    let _selected_chat_id = Some(123);
 
     // Пользователь нажал Esc
     let selected_chat_id: Option<i64> = None;
@@ -97,7 +97,7 @@ async fn test_scroll_messages_in_chat() {
 
     let client = client.with_messages(123, messages);
 
-    let msgs = client.get_messages(123);
+    let _msgs = client.get_messages(123);
 
     // Скролл начинается снизу (последнее сообщение видно)
     let mut scroll_offset: usize = 0;