feat(frontend): add toast notification system and transaction history list (Closes #48, #52)

[ToryMic]

Summary

Resolves two good first issue frontend tickets:

• [Frontend] Add toast notification system for success and error feedback #48 — Toast notification system: ToastProvider + useToast() hook so any component can trigger success / error / info toasts. Toasts render in a fixed bottom-right stack, auto-dismiss after 4 seconds, and use the project CSS design tokens (dark/light compatible).
• [Frontend] Add transaction history list component #52 — Transaction history list: typed <TransactionList /> that renders type icon, truncated ID, amount, formatted date, and a status badge (pending/confirmed/failed), with a friendly empty state.

Changes

• frontend/src/components/Toast.tsx — ToastProvider, presentational Toast, toast store context (useToastContext), AUTO_DISMISS_MS = 4000
• frontend/src/components/Toast.module.css — fixed bottom-right viewport, per-type color/icon styling, entry animation, responsive
• frontend/src/hooks/useToast.ts — useToast() → { toast, success, error, info, dismiss, clear }
• frontend/src/components/TransactionList.tsx — typed Transaction / TransactionType / TransactionStatus, TransactionList, EmptyState
• frontend/src/components/TransactionList.module.css — row/empty-state styling with design tokens
• frontend/src/components/index.ts — exports the new components/types
• frontend/src/main.tsx — wraps <App /> in <ToastProvider>
• frontend/src/App.tsx — triggers toasts on connect/deposit/transfer/withdraw (replaces inline success banner) and demos the transaction list

Verification

• cd frontend && npm install && npm run build — passes (tsc && vite build)

Notes

• Issue [Frontend] Add dark/light mode toggle with persisted preference #54 (theme toggle) already existed in the tree (useTheme.ts + Header toggle wired in App.tsx), so I picked [Frontend] Add toast notification system for success and error feedback #48 and [Frontend] Add transaction history list component #52 instead.