[ML 0.1/UI.04.6] - Add account indicator visibility for unified folders

[rafaeltonholo]

Blocked by

• [ML 0.1/UI.04.3] - Add unified folder support #10776

Description

Show the account colour indicator on message items when viewing a unified folder, so users can distinguish which account each message belongs to.

Acceptance Criteria

• Account indicator is visible when the current folder is a unified-account folder.
• Account indicator is hidden for single-account folder views.
• Visibility is determined based on the folder's account ID in SetupGlobalState.