feat(elections): accessible ranked-choice ballot UI

Summary

Build the voter-facing ballot: a mobile-friendly, WCAG 2.2 AA-compliant interface for ranking candidates in each race.

Part of #1933.

Requirements

• Per-race ranking UI that supports drag-to-reorder and keyboard-only reordering (up/down arrow keys)
• Screen-reader-accessible announcements when rank changes
• Support partial ballots (voter may rank a subset of candidates)
• Support skipping a race (explicit "I don't wish to vote in this race" option)
• Candidate cards show name, bio, optional photo — never anything that could hint at voter preference
• Review screen summarizing all rankings before final submission
• Clear confirmation of submission with no way to edit afterward
• Works on mobile viewports (375×667 minimum); touch-friendly rank targets
• Passes axe-core automated accessibility checks in CI

Context

• Epic: #1933
• Depends on schema issue

Implementation notes

• Use the design-system's focus/contrast tokens — do not introduce new color tokens
• Consider a "rank" number + candidate list pattern (select a rank for each candidate) as a keyboard-friendly alternative to pure drag — prototype both and pick the more accessible one
• Run through the `uiux-design-team:accessibility-audit` skill before calling this done

No implementation files, tests, or entry points are named. Start by resolving the schema dependency and reviewing the design-system focus and contrast tokens; prototype the rank-number and candidate-list pattern alongside drag-to-reorder. Done means the ballot meets the listed ranking, review, submission, mobile, keyboard, screen-reader, and axe-core requirements and passes the accessibility-audit skill.