Mark files as viewed during review

## Feature Request

No way to mark individual files as "viewed" when reviewing a changeset, so files lose track of review progress after navigating away and editing.

**Use case:** Reviewing a multi-file changeset. You open a file, review it, make an edit, then move on. When you return to the file list there is no indication which files you have already looked at or touched — you re-review files you already cleared and lose your place in large changesets.

**Proposed behavior:**
- Per-file "viewed" checkbox/indicator in the Changes file tree (GitHub PR-style "Viewed" toggle)
- Viewed state visually distinct (dimmed, checkmark) so unreviewed files stand out
- Auto-reset a file's viewed state if it changes again after being marked viewed
- Viewed count / progress indicator for the changeset (e.g. "3/8 viewed")