Browsable folder tree for imported mailboxes (Inbox / Sent / sub-folders)

Surface the original mailbox folder structure as a navigable tree in the project view, so each mailbox can be explored by its folders (Inbox, Sent, Deleted, Archive and custom sub-folders) separately rather than as one flat list.

The folder path is already captured per email on import, so this is primarily a UI/navigation feature.

Raised while reviewing a large multi-mailbox eDiscovery import (~15.8k emails across 20 PST parts).