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>

src/input/handlers/chat.rs

@@ -7,7 +7,11 @@
 //! - Loading older messages
 
 use crate::app::App;
-use crate::tdlib::{TdClientTrait, ChatAction, ReplyInfo};
+use crate::app::methods::{
+    compose::ComposeMethods, messages::MessageMethods,
+    modal::ModalMethods, navigation::NavigationMethods,
+};
+use crate::tdlib::{TdClientTrait, ChatAction};
 use crate::types::{ChatId, MessageId};
 use crate::utils::{is_non_empty, with_timeout, with_timeout_msg};
 use crate::input::handlers::{copy_to_clipboard, format_message_for_clipboard};