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/config/mod.rs

@@ -1,11 +1,10 @@
 pub mod keybindings;
 
-use crossterm::event::KeyCode;
 use serde::{Deserialize, Serialize};
 use std::fs;
 use std::path::PathBuf;
 
-pub use keybindings::{Command, KeyBinding, Keybindings};
+pub use keybindings::{Command, Keybindings};
 
 /// Главная конфигурация приложения.
 ///
@@ -347,8 +346,6 @@ impl Config {
     /// API_HASH=your_api_hash_here
     /// ```
     pub fn load_credentials() -> Result<(i32, String), String> {
-        use std::env;
-
         // 1. Пробуем загрузить из ~/.config/tele-tui/credentials
         if let Some(credentials) = Self::load_credentials_from_file() {
             return Ok(credentials);
@@ -423,7 +420,7 @@ impl Config {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crossterm::event::{KeyEvent, KeyModifiers};
+    use crossterm::event::{KeyCode, KeyEvent, KeyModifiers};
 
     #[test]
     fn test_config_default_includes_keybindings() {