Allow admins to edit their vote after submitting a review

[balebbae]

Admins can only cast a vote once through the grading flow. Once a vote is submitted the review moves to the Completed tab and there is no way to change it, so a misclick or a second look at an application leaves the vote stuck.

Goal

Let an admin re-open and edit a review they have already voted on — vote, travel vote, and notes.

Notes

• The backend already allows it: PUT /v1/admin/reviews/{reviewID} (submitVote in cmd/api/reviews.go → ApplicationReviewsStore.SubmitVote) updates by review ID + admin ID with no vote IS NULL guard, so re-submitting overwrites the existing vote and bumps reviewed_at. Confirm that's intentional and that nothing downstream (review stats, decision emails) assumes a vote is final.
• The gap is mostly frontend, in client/portal/src/pages/admin/reviews/: the Completed tab (components/ReviewsTable.tsx) needs an edit affordance that opens the grading panel (grading/GradingPage.tsx / GradingVotingPanel.tsx) prefilled with the existing vote, travel vote, and notes.
• Only the admin who owns the review may edit it — already enforced by the admin_id scope on the update, but the UI should not offer editing on someone else's review.