Rules-based subfoldering (sender/subject keyword) #17

Sprint: sprint-6 (files: Settings.swift, AttachmentUnloader.swift)

Extend AttachmentUnloader's year/type organization with sender- or subject-keyword-based subfolders, chosen via a Settings-backed rule list.

Acceptance criteria

• A message matching a configured keyword saves into the configured subfolder instead of the default year/type folder.
• Non-matching messages fall back to today's default organization.