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>
ec2758ce18