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/chat_list.rs

@@ -177,10 +177,10 @@ fn snapshot_chat_with_online_status() {
     // Устанавливаем онлайн-статус для чата напрямую
     let chat_id = ChatId::new(123);
     let user_id = tele_tui::types::UserId::new(123);
-    
+
     // Регистрируем чат как приватный
-    app.td_client.user_cache.register_private_chat(chat_id, user_id);
-    
+    app.td_client.user_cache.chat_user_ids.insert(chat_id, user_id);
+
     // Устанавливаем онлайн-статус
     app.td_client.user_cache.user_statuses.insert(user_id, UserOnlineStatus::Online);