refactor: cleanup unused code and warnings

Comprehensive cleanup of unused methods, dead code, and compiler warnings:

## Removed Methods (15):
- Duplicate delegation methods: is_authenticated, get_typing_text, get_user_name from TdClient
- Obsolete methods: TdClient::init() (duplicated in main.rs)
- Unused getters: UserCache::{get_username, get_name, get_user_id_by_chat}
- Unused builder methods: MessageBuilder::{edited, add_reaction}
- Unused utility: ChatState::is_normal()
- Dead code: HotkeysConfig::{matches, key_matches} (kept for tests)
- Unused method: UserCache::register_private_chat()
- Getter replaced with direct field access: MessageInfo::edit_date()

## Removed Module:
- error.rs - Unused error handling module (TeletuiError, ErrorVariant, IntoTeletuiError)

## Removed Constants (8):
- EMOJI_PICKER_COLUMNS, EMOJI_PICKER_ROWS, MAX_INPUT_HEIGHT
- MIN_TERMINAL_WIDTH, MIN_TERMINAL_HEIGHT
- TDLIB_CHAT_LIMIT, MAX_USERNAME_DISPLAY_LENGTH, MESSAGE_TEXT_INDENT

## Fixed Warnings:
- Removed unused imports (8 instances)
- Fixed unreachable patterns (10 instances)
- Fixed irrefutable if let patterns (2 instances)
- Fixed unused variables (1 instance)
- Removed dead_code annotations where appropriate

## Improvements:
- Integrated Config::load_credentials() into TdClient::new() for better credential management
- Replaced edit_date() getter with direct field access (message.metadata.edit_date)
- Updated tests to use direct field access instead of removed getters

## Test Results:
All tests passing: 499 passed, 0 failed

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

tests/edit_message.rs

@@ -39,15 +39,15 @@ async fn test_edit_message_sets_edit_date() {
     // Получаем дату до редактирования
     let messages_before = client.get_messages(123);
     let date_before = messages_before[0].date();
-    assert_eq!(messages_before[0].edit_date(), 0); // Не редактировалось
+    assert_eq!(messages_before[0].metadata.edit_date, 0); // Не редактировалось
 
     // Редактируем сообщение
     client.edit_message(ChatId::new(123), msg.id(), "Edited".to_string()).await.unwrap();
 
     // Проверяем что edit_date установлена
     let messages_after = client.get_messages(123);
-    assert!(messages_after[0].edit_date() > 0);
-    assert!(messages_after[0].edit_date() > date_before); // edit_date после date
+    assert!(messages_after[0].metadata.edit_date > 0);
+    assert!(messages_after[0].metadata.edit_date > date_before); // edit_date после date
 }
 
 /// Test: Редактирование только своих сообщений (проверка через can_be_edited)