feat(#41): add toast notification system for transaction status

[JosephOnuh]

Closes #41

Changes

• Add ToastContext.tsx with ToastProvider, useToast hook, and toast UI
  • Three types: pending (blue), success (green), error (red)
  • Auto-dismiss after 5 s for success/error; pending stays until explicitly dismissed
  • Includes explorer link for transaction hashes (View tx ↗)
  • Accessible: role=region, aria-live=polite, role=alert per toast
• Wrap app in <ToastProvider> in main.tsx
• Add connect() wallet function in App.tsx demonstrating toast usage:
  • Shows "Connecting wallet..." pending toast
  • Resolves to success or error toast on completion
• Add explorerTxUrl / explorerAccountUrl helpers to config.ts

Testing

• Click Connect Wallet → pending toast appears
• On success/failure → toast updates with correct type and message
• Transaction toasts include a link to the Stellar explorer

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