Implement off-chain voting

## Outcome

Users can participate in verifiable off-chain voting based on the approved security and product design.

## Scope

- Implement eligibility, ballot, tally, and result-verification flows
- Implement required privacy, audit, and abuse controls
- Integrate with profiles and proposal context
- Complete security review, accessibility validation, and operational testing

## Acceptance criteria

- [ ] Scope is decomposed into independently deliverable child issues.
- [ ] Architecture and API decisions are linked from this issue.
- [ ] Test, documentation, observability, rollout, and failure behavior are defined where applicable.
- [ ] Epic exit criteria are verified before closure.

## Planning note

This is a 2026 GovTool revamp work item. Its child features and tasks should be added as the scope is refined.

No files, tests, or entry points are named. Start by decomposing the eligibility, ballot, tally, result-verification, privacy, audit, abuse-control, profile, and proposal-context work into child issues, then link the architecture and API decisions; done means the epic exit criteria and required validation plans are verified.

Written by the indexing model from the issue text.