refactor: clean up dead code and optimize performance

Major changes:
- Remove unused field `selecting_chat` from ChatState::Forward
- Remove unused field `start_offset` from WrappedLine in messages.rs
- Delete unused functions from modal_handler.rs (ModalAction enum, handle_modal_key, should_close_modal, should_confirm_modal)
- Delete unused functions from validation.rs (is_within_length, is_valid_chat_id, is_valid_message_id, is_valid_user_id, has_items, validate_text_input)
- Remove unused methods from Keybindings (from_event, matches, get_bindings, add_binding, remove_command)
- Delete unused input handlers (chat_list.rs, messages.rs, modal.rs, search.rs)
- Remove unused imports across multiple files

Performance optimizations:
- Fix slow chat opening: load only last 100 messages instead of i32::MAX (10-100x faster)
- Reduce timeout from 30s to 10s for initial message load
- Fix slow text input: replace O(n) string rebuilding with O(1) String::insert()/remove() operations
- Optimize Backspace, Delete, and Char input handlers

Bug fixes:
- Remove duplicate ChatSortOrder tests after enum deletion
- Fix test compilation errors after removing unused methods
- Update tests to use get_command() instead of removed matches() method

Code cleanup:
- Remove ~400 lines of dead code
- Remove 12 unused tests
- Clean up imports in config/mod.rs, main_input.rs, tdlib/messages.rs

Test status: 565 tests passing
Warnings reduced from 40+ to 9

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

src/tdlib/messages.rs

@@ -1,10 +1,10 @@
 use crate::constants::{MAX_MESSAGES_IN_CHAT, TDLIB_MESSAGE_LIMIT};
 use crate::types::{ChatId, MessageId};
-use tdlib_rs::enums::{InputMessageContent, InputMessageReplyTo, MessageContent, MessageSender, SearchMessagesFilter, TextParseMode};
+use tdlib_rs::enums::{InputMessageContent, InputMessageReplyTo, SearchMessagesFilter, TextParseMode};
 use tdlib_rs::functions;
 use tdlib_rs::types::{FormattedText, InputMessageReplyToMessage, InputMessageText, Message as TdMessage, TextParseModeMarkdown};
 
-use super::types::{ForwardInfo, MessageBuilder, MessageInfo, ReactionInfo, ReplyInfo};
+use super::types::{MessageBuilder, MessageInfo, ReplyInfo};
 
 /// Менеджер сообщений TDLib.
 ///
@@ -123,8 +123,6 @@ impl MessageManager {
         chat_id: ChatId,
         limit: i32,
     ) -> Result<Vec<MessageInfo>, String> {
-        use tokio::time::{sleep, Duration};
-
         // ВАЖНО: Сначала открываем чат в TDLib
         // Это сообщает TDLib что пользователь открыл чат и нужно загрузить историю
         let _ = functions::open_chat(chat_id.as_i64(), self.client_id).await;