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/input/handlers/mod.rs

@@ -1,26 +1,14 @@
-//! Input handlers organized by screen/mode
+//! Input handlers organized by functionality
 //!
 //! This module contains handlers for different input contexts:
 //! - global: Global commands (Ctrl+R, Ctrl+S, etc.)
-//! - profile: Profile mode input
-//! - search: Search modes (chat search, message search)
-//! - modal: Modal modes (pinned, reactions, delete, forward)
-//! - messages: Message input when chat is open
-//! - chat_list: Chat list navigation
 //! - clipboard: Clipboard operations
+//! - profile: Profile helper functions
 
-pub mod chat_list;
 pub mod clipboard;
 pub mod global;
-pub mod messages;
-pub mod modal;
 pub mod profile;
-pub mod search;
 
-// pub use chat_list::*;  // Пока не используется
 pub use clipboard::*;
 pub use global::*;
-// pub use messages::*;   // Пока не используется
-// pub use modal::*;      // Пока не используется
-pub use profile::get_available_actions_count;  // Используется в main_input
-// pub use search::*;     // Пока не используется
+pub use profile::get_available_actions_count;