feat(frontend): add revoke vote button to active proposals

[omolobamoyinoluwa-max]

Summary

Admins can call revoke_vote on the contract but there is no UI for it. Add a Revoke button to each proposal card in the dashboard.

Acceptance criteria

• Revoke button visible only if connected wallet has voted
• Calls /api/freeze/build with revoke_vote operation
• Proposal vote count updates after revoke