40: Implement error/offline states and the Toast system

[Saber5656]

Derived from docs/issues/40-error-offline-states.md — the canonical source of truth. Execution order & dependencies: docs/ISSUE_PLAN.md (PR #1). If this issue and the doc disagree, the doc wins.

Summary

Build the final Toast component and wire the DESIGN §13 degraded-state matrix: offline detection,
basemap failure notice, POI error pill with retry, and consistent timeout policy — replacing
interim toasts from earlier issues.

Context

DESIGN §13 defines exact UX per failure. ADR-001 means upstream outages surface directly to the
client; calm, specific messaging is the difference between "broken" and "degraded".

Scope

• src/ui/components/Toast.ts (final form; replaces 31's inline stub): single-slot queue (max 3
  pending, drop-oldest), kinds info/error, optional action button (used by 31's update toast),
  auto-dismiss 4 s (action toasts 8 s), aria-live="polite", reduced-motion aware.
• Offline handling: navigator.onLine + online/offline events → store flag ui.offline (add
  to AppState; update 09 types): one-time toast net.offline (オフラインのようです — 地図 データは読み込めません) on transition; badge dot on MenuButton while offline; net.backOnline
  info toast on recovery.
• Basemap tile failures (non-404 network errors > 10 within 10 s while online) → single toast
  net.tilesFailing (rate-limited: once per 5 min) — implement counter in mapController error
  listener (10).
• POI error pill (map-region top, below CoverageBanner): shown when poi.status==='error':
  i18n poi.fetchError (記事を取得できませんでした) + common.retry button → re-trigger
  fetch (27 exposes retry()).
• Verify timeout policy: all fetches already use AbortSignal.timeout(8000) (24) — add a lint-
  level grep test (unit spec asserting no raw fetch( outside wikimediaClient/allowed files).

Detailed Requirements

1. Toasts never stack visually (queue), never block map gestures, respect safe areas.
2. Offline flag must not disable UI (everything stays tappable; actions fail soft).
3. No auto-retry loops anywhere (manual retry only; §13).
4. Past-layer 404s remain silent (regression guard: e2e from 18/21 unaffected).

Acceptance Criteria

• e2e: context.setOffline(true) → offline toast once + menu dot; setOffline(false) →
  recovery toast; no repeat spam on flapping (3 rapid toggles ≤ 2 offline toasts —
  hysteresis 5 s).
• e2e: POI 500 stub → pill + retry works (second stub succeeds) → pill clears.
• e2e: update-toast action path from 31 still works with final Toast.
• Unit: toast queue (order, drop-oldest, action timeout), offline hysteresis, tile-failure
  rate limiter (fake timers).

Validation

tests/e2e/errors.spec.ts + unit specs.

Dependencies

10, 19, 24, 27, 31 (integrations it finalizes). Updates 09 (state field).

Non-goals

Error telemetry (never in v1), Sentry-like reporting, offline map data.

Design References

DESIGN §13, §8 #12; ADR-001, ADR-005.