refactor: consolidate message loading logic into chat_loader.rs

Move all three phases of chat message loading from scattered locations
into a single dedicated module for better cohesion and navigability:
- Phase 1: open_chat_and_load_data (from handlers/chat_list.rs)
- Phase 2: process_pending_chat_init (extracted from 70-line inline block in main.rs)
- Phase 3: load_older_messages_if_needed (from handlers/chat.rs)

No behaviour changes — pure refactoring.

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

src/input/handlers/mod.rs

@@ -6,12 +6,14 @@
 //! - profile: Profile helper functions
 //! - chat: Keyboard input handling for open chat view
 //! - chat_list: Navigation and interaction in the chat list
+//! - chat_loader: All phases of chat message loading
 //! - compose: Text input, editing, and message composition
 //! - modal: Modal dialogs (delete confirmation, emoji picker, etc.)
 //! - search: Search functionality (chat search, message search)
 
 pub mod chat;
 pub mod chat_list;
+pub mod chat_loader;
 pub mod clipboard;
 pub mod compose;
 pub mod global;
@@ -19,6 +21,7 @@ pub mod modal;
 pub mod profile;
 pub mod search;
 
+pub use chat_loader::{load_older_messages_if_needed, open_chat_and_load_data, process_pending_chat_init};
 pub use clipboard::*;
 pub use global::*;
 pub use profile::get_available_actions_count;