[ML 0.1/UI.04.3] - Add unified folder support

[rafaeltonholo]

Blocked by

• [ML 0.1/UI.04.2] - Implement legacy message list loading bridge #10775

Description

Add support for unified folder (all messages from all accounts) in the Compose message list. This includes updating the LoadFolderInformationSideEffect to handle the unified folder case and adjusting the MessageItemAvatar component.

Acceptance Criteria

• LoadFolderInformationSideEffect correctly loads folder information for unified folders.
• MessageItemAvatar renders correctly in unified folder context.
• No regressions for regular (non-unified) folder display.