feat: integrate Sonner toast system with Toaster component and global API error interceptor (Fixes #19)

[waterWang]

Summary

Implements issue #19 — Toast Notification System for Payment & Error Events.

The codebase already had a NotificationProvider (powered by Sonner) plus notify / notifySuccess / notifyError calls wired into 11+ components and providers (wallet connection, socket events, payment submission, contract invocation, clipboard, bulk payment tracking, network switch). However, Sonner's <Toaster /> component was never rendered, so every toast call was silently invisible to users.

Changes

1. frontend/src/main.tsx — Render Sonner's <Toaster /> (top-right, rich colors, close button, 4s duration, max 5 toasts) inside ThemeProvider so all existing notification calls become visible. Styled to match the app's dark design system via toast-notification class.

2. frontend/src/services/apiErrorInterceptor.ts (new) — Global Axios response interceptor that converts HTTP 4xx/5xx failures into actionable error toasts:

   • Friendly status-specific messages (401 session expired, 403 permission, 429 rate limit, 5xx server errors, etc.)
   • Surfaces the backend's message field when available
   • Skips noise: localhost/dev requests, auth-endpoint 401s (handled by AuthProvider), and /api/contracts (has its own retry logic)

Acceptance Criteria

• Sonner toast system integrated — <Toaster /> mounted so notifications actually render
• Success/Error notification patterns established — notifySuccess / notifyError used across wallet, payment, contract, and API layers
• Notifications provide actionable info — payment tx hashes, wallet addresses, specific backend error messages

Verification

• npx tsc --noEmit passes (exit 0)
• Existing tests still pass (Needs CI to confirm)

Fixes #19