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/e2e_user_journey.rs

@@ -293,7 +293,7 @@ async fn test_user_journey_edit_during_conversation() {
     let edited_history = client.get_chat_history(ChatId::new(555), 50).await.unwrap();
     assert_eq!(edited_history.len(), 1);
     assert_eq!(edited_history[0].text(), "I'll be there at 5pm tomorrow");
-    assert!(edited_history[0].edit_date() > 0, "Должна быть установлена дата редактирования");
+    assert!(edited_history[0].metadata.edit_date > 0, "Должна быть установлена дата редактирования");
 
     // 6. Проверяем историю редактирований
     assert_eq!(client.get_edited_messages().len(), 1);