Track viewed-file state during revision review #388

This request is about review progress state: marking files as viewed/unviewed while stepping through a large diff.

requested behavior:

Mark current file as viewed.
Mark current file as unviewed.
Toggle viewed state from the file tree/explorer.
Persist viewed state for the current diff/revision range.
Visually distinguish viewed vs unviewed files in the explorer.
Preserve viewed state when refreshing the same review range.
Support marking a file viewed even when no further action is needed for that file.