[Vetting] Add “Next unreviewed file” navigation

[aashish24]

Background:
Reviewers will review over multiple sessions and need a fast way to continue.

Requirements (Data Task Checklist):

• Provide a Next Unreviewed action that takes the reviewer to the next file that is Not Completed.
• Skip files that are already marked as Completed.
• If all files are completed, show an explicit completion message.

Acceptance criteria:

• Next Unreviewed skips completed files.
• When no unreviewed files remain, the user sees "All reviews complete".