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/ui/mod.rs

@@ -1,10 +1,16 @@
+//! UI rendering module.
+//!
+//! Routes rendering by screen (Loading → Auth → Main) and checks terminal size.
+
 mod auth;
 pub mod chat_list;
+mod compose_bar;
 pub mod components;
 pub mod footer;
 mod loading;
 mod main_screen;
 pub mod messages;
+mod modals;
 pub mod profile;
 
 use crate::app::{App, AppScreen};