Add accessible toast queue with role=status (#458)

[Dickson2015]

Summary

Closes #458 — Add accessible toast queue with role=status.

A single, centralized toast queue for transient status messages. Mounted once in App.tsx inside NotificationProvider, it renders every active toast in a role="status" live region — WCAG 2.1 SC 4.1.3 (Status Messages).

What's included

• src/components/ToastContainer.tsx (new) — centralized queue component mounted in App.tsx; outer container uses role="status" + aria-live="polite" + aria-atomic="true" + aria-label="Notifications"
• src/components/notifications/ToastContainer.tsx — ToastItem presentational component with per-severity semantics: success/info/warning → role="status" + aria-live="polite"; error/danger → role="alert" + aria-live="assertive"
• src/hooks/useToast.ts — typed convenience helpers (success, error, warning, info, dismiss)
• docs/TOAST_QUEUE.md (new) — API, ARIA semantics, visible behaviour, files, and test coverage; linked from the README

Fixes included on this branch

• NotificationCenter TDZ crash — the Shift+R shortcut effect referenced a const useCallback before initialization (ReferenceError on every render); moved below handleMarkAllAsRead so the dep array can reference it safely
• Shift+R case-sensitivity — handler compared e.key === 'r' but Shift+R reports 'R'; normalized with toLowerCase() so the shortcut actually works
• NotificationBell — restored the label prop and one-shot bell-pulse on danger/error arrivals (feature dropped in an earlier rewrite but still covered by tests); pulse timer held in a ref and cleared on unmount only, so a mid-animation notifications change can't stick the pulse; prefers-reduced-motion: reduce fallback added
• CollateralSwap import — restored import CollateralSwap from "./pages/CollateralSwap" in App.tsx (dropped accidentally while wiring in ToastContainer, breaking the /collateral-swap route)
• Tests — added ToastContainer suite (17 tests), per-severity role tests, sticky-pulse regression test, contrast tests; removed an unused binding

Validation

• Toast/notification suite: 164 tests passing across repeated runs (ToastContainer, notifications/ToastContainer, UndoToast, NotificationCenter, NotificationBell, __tests__/NotificationCenter, safe-area)
• tsc -b introduces no new errors in touched files

Note (pre-existing, not from this branch): App.test.tsx / App.credit-lines-route.test.tsx have failures on the base commit afcb629 and on upstream/main too (duplicate WalletReconnectBanner/NetworkMismatchBanner imports and missing props in App.tsx from an earlier merge). They are unrelated to this branch and identical at the merge-base.

Accessibility Checklist

• Keyboard navigation works (Tab, Shift+Tab, Enter, Escape)
• Focus indicators are clearly visible (2px outline, 2px offset)
• Contrast ratios meet WCAG AA (4.5:1 text, 3:1 large text/icons) — see ToastContainer.contrast.test.ts
• Touch targets are at least 44×44 px
• Semantic HTML and ARIA roles/labels are used (role="status" / role="alert", aria-live, aria-atomic, aria-label)
• prefers-reduced-motion is respected