[Vetting] Add “Mark Complete / Submit Review” per file (per user) #277

Background:
Reviewers need an explicit action to indicate that they have completed reviewing a file.

Requirements (Data Task Checklist):

• Add a Mark Complete / Submit action for the current user on the currently open file.
• Persist completion state on a per-user, per-file basis.
• Supported states: Not Completed and Completed.
• After marking a file as completed, allow the reviewer to reopen the file for comparison or reference.
• Ensure completed files remain clearly marked as completed in the UI.

Acceptance criteria:

• A reviewer can mark a file as completed.
• The UI displays the completion state for the current reviewer and file.
• A reviewer can reopen files that are marked as completed.