Show unread indicator on documents that haven't been opened

Summary
Outline has no way to distinguish between documents a user has opened and ones they haven't. When new documents are added to a collection, there's no visual indicator to flag them as unread, so users have to rely on memory or check timestamps to find new content.

User Story
As a user, I want unread documents to be visually marked so I can quickly identify content I haven't read yet, without having to scan through timestamps manually.

Rough Scope: Medium

Source: Outline — document list / collection sidebar