refactor: complete Phase 13 deep architecture refactoring (etaps 3-7)

Split monolithic files into modular architecture:
- ui/messages.rs (893→365 lines): extract modals/, compose_bar.rs
- tdlib/messages.rs (836→3 files): split into messages/mod, convert, operations
- config/mod.rs (642→3 files): extract validation.rs, loader.rs
- Code duplication cleanup: shared components, ~220 lines removed
- Documentation: PROJECT_STRUCTURE.md rewrite, 16 files got //! docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/helpers/app_builder.rs

@@ -283,6 +283,7 @@ impl TestAppBuilder {
 mod tests {
     use super::*;
     use crate::helpers::test_data::create_test_chat;
+    use tele_tui::app::methods::messages::MessageMethods;
 
     #[test]
     fn test_builder_defaults() {

tests/input_navigation.rs

@@ -7,6 +7,7 @@ mod helpers;
 use crossterm::event::{KeyCode, KeyEvent, KeyModifiers};
 use helpers::app_builder::TestAppBuilder;
 use helpers::test_data::{create_test_chat, TestMessageBuilder};
+use tele_tui::app::methods::messages::MessageMethods;
 use tele_tui::input::handle_main_input;
 
 fn key(code: KeyCode) -> KeyEvent {