docs: add rustdoc comments for public API (P4.12 partial)

- Add comprehensive documentation for TdClient:
  * Struct-level docs with examples
  * Authentication methods (send_phone_number, send_code, send_password)
  * Chat methods (load_chats, load_folder_chats, leave_chat, get_profile_info)
  * All methods now have parameter docs, return types, and error descriptions

- Add comprehensive documentation for App:
  * Struct-level docs with state machine explanation
  * Constructor documentation
  * Examples for common usage patterns

- Progress: +60 doc comment lines (210 → 270)
- Update REFACTORING_ROADMAP.md (P4.12 partial completion)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -8,6 +8,41 @@ use crate::tdlib::{ChatInfo, TdClient};
 use crate::types::{ChatId, MessageId};
 use ratatui::widgets::ListState;
 
+/// Main application state for the Telegram TUI client.
+///
+/// Manages all application state including authentication, chats, messages,
+/// and UI state. Integrates with TDLib через `TdClient` and handles user input.
+///
+/// # State Machine
+///
+/// The app uses a type-safe state machine (`ChatState`) for chat-related operations:
+/// - `Normal` - default state
+/// - `MessageSelection` - selecting a message
+/// - `Editing` - editing a message
+/// - `Reply` - replying to a message
+/// - `Forward` - forwarding a message
+/// - `DeleteConfirmation` - confirming deletion
+/// - `ReactionPicker` - choosing a reaction
+/// - `Profile` - viewing profile
+/// - `SearchInChat` - searching within chat
+/// - `PinnedMessages` - viewing pinned messages
+///
+/// # Examples
+///
+/// ```no_run
+/// use tele_tui::app::App;
+/// use tele_tui::config::Config;
+///
+/// let config = Config::default();
+/// let mut app = App::new(config);
+///
+/// // Navigate through chats
+/// app.next_chat();
+/// app.previous_chat();
+///
+/// // Open a chat
+/// app.select_chat();
+/// ```
 pub struct App {
     pub config: crate::config::Config,
     pub screen: AppScreen,
@@ -42,6 +77,18 @@ pub struct App {
 }
 
 impl App {
+    /// Creates a new App instance with the given configuration.
+    ///
+    /// Initializes TDLib client, sets up empty chat list, and configures
+    /// the app to start on the Loading screen.
+    ///
+    /// # Arguments
+    ///
+    /// * `config` - Application configuration loaded from config.toml
+    ///
+    /// # Returns
+    ///
+    /// A new `App` instance ready to start authentication.
     pub fn new(config: crate::config::Config) -> App {
         let mut state = ListState::default();
         state.select(Some(0));