feat(errors): integrate a crash reporter (decision needed: Sentry vs Crashlytics)

## Problem
(verified) Zero crash/error telemetry (pubspec.yaml has no reporting SDK). Open issues like #443 ("User gets logged out") are currently undiagnosable in the field.

## Proposed fix
Recommend `sentry_flutter` (EU data residency / self-host option fits health-data governance better than Crashlytics) with `beforeSend` scrubbing, no default PII, release + dSYM upload in CI (ref #585).

**Decision needed** on vendor & hosting before implementation.

## Acceptance criteria
- [ ] Release-build crash visible in dashboard with symbolicated stack
- [ ] Event payloads contain no user ID/username/token (verified by inspection)
- [ ] Documented opt-out posture w.r.t. study ethics

**Effort:** M · **Priority:** P1 · **Change risk:** Low