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/notifications.rs

@@ -269,7 +269,7 @@ mod tests {
     #[test]
     fn test_notification_manager_creation() {
         let manager = NotificationManager::new();
-        assert!(manager.enabled);
+        assert!(!manager.enabled); // disabled by default
         assert!(!manager.only_mentions);
         assert!(manager.show_preview);
     }