Add an untimed accessible practice mode

## Priority

P2 — focused reliability or product improvement

## Context

Portfolio audit follow-up (2026-09-08). The portfolio recommendation is a focused practice workflow that helps learners who need more time. Existing speed-based stars and points should not make accessible practice feel like failure. This extends the prior accessibility work rather than reopening it without evidence.

## Starting points / evidence

Start with `src/AppReducer.ts` calculatePoints/calculateStars and existing accessibility settings. Coordinate with #166 for progress reporting.

## Implementation scope

Expose an untimed practice option in the existing mode/settings UI. Remove speed pressure and time penalties in that mode while retaining deterministic answer feedback and skill practice. Ensure keyboard/screen-reader operation and reduced-motion preferences work through a complete session.

## Acceptance criteria

- [ ] Untimed mode is visible, persists and can be changed without losing progress.
- [ ] Equal correct answers receive equivalent practice credit regardless of response time.
- [ ] Timed and untimed records are distinguishable so analytics do not compare incompatible scores.
- [ ] Keyboard-only completion and screen-reader feedback are manually checked and documented.
- [ ] No essential information depends solely on color, animation or audio.

Keep the change focused on this issue. Use synthetic fixtures for sensitive data and mocked providers for automated tests; do not require paid model calls to run the regression suite.

Start with src/AppReducer.ts, especially calculatePoints/calculateStars, then trace the existing accessibility settings and mode/settings UI. Coordinate progress reporting with #166 and verify the complete session using keyboard-only and screen-reader checks. Done means untimed progress persists, avoids time-based penalties, remains distinguishable from timed records, and meets the stated accessibility criteria.

Written by the indexing model from the issue text.