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/app/mod.rs

@@ -1,7 +1,12 @@
+//! Application state module.
+//!
+//! Contains `App<T>` — the central state struct parameterized by `TdClientTrait`
+//! for dependency injection. Methods are organized into trait modules in `methods/`.
+
 mod chat_filter;
 mod chat_state;
 mod state;
-mod methods;
+pub mod methods;
 
 pub use chat_filter::{ChatFilter, ChatFilterCriteria};
 pub use chat_state::ChatState;
@@ -9,7 +14,7 @@ pub use state::AppScreen;
 pub use methods::*;
 
 use crate::tdlib::{ChatInfo, TdClient, TdClientTrait};
-use crate::types::{ChatId, MessageId};
+use crate::types::ChatId;
 use ratatui::widgets::ListState;
 
 /// Main application state for the Telegram TUI client.
@@ -35,6 +40,7 @@ use ratatui::widgets::ListState;
 ///
 /// ```no_run
 /// use tele_tui::app::App;
+/// use tele_tui::app::methods::navigation::NavigationMethods;
 /// use tele_tui::config::Config;
 ///
 /// let config = Config::default();