feat: Add escrow template creation & management system

[swankybae]

Description

This PR implements three major enhancements to the escrow system:

1. Escrow Template Creation & Management:

   • Built-in templates (Freelance Payment, Product Purchase, Service Agreement, Custom)
   • Custom template storage in localStorage
   • Template CRUD operations via /settings/templates page
   • Recent used templates in selection screen

2. Centralized Toast Notification System:

   • Success/Error/Info/Warning variants with distinct icons/colors
   • Queue management (max 3 visible toasts)
   • Deduplication of identical messages within 5 seconds
   • Persistent error toasts, 5s auto-dismiss for others
   • Action button support in toasts
   • "Dismiss All" button when ≥2 toasts are visible
   • Accessibility (keyboard dismiss, ARIA labels)
   • All alert() calls replaced with toasts

3. Deadline Countdown Timer:

   • Real-time countdown with days/hours/minutes/seconds
   • Color-coded urgency: Green (>48h), Yellow (24-48h), Red (<24h), Expired
   • Updates every second when <1h, every minute otherwise
   • "Request Refund" button for expired escrows
   • Full datetime with user timezone on hover
   • Countdown pauses during disputes
   • Browser notifications at 24h and 1h warnings
   • Handles already-expired escrows on page load

Files Modified

• New: apps/frontend/lib/templates.ts
• New: apps/frontend/hooks/useTemplates.ts
• New: apps/frontend/components/escrow/create/TemplateSelector.tsx
• New: apps/frontend/app/settings/templates/page.tsx
• New: apps/frontend/hooks/useCountdown.ts
• New: apps/frontend/hooks/useCountdown.test.ts
• New: apps/frontend/components/escrow/CountdownTimer.tsx
• Modified: apps/frontend/app/contexts/ToastContext.tsx
• Modified: apps/frontend/app/contexts/ToastProvider.tsx
• Modified: apps/frontend/components/ui/Toast.tsx
• Modified: apps/frontend/components/ui/ToastContainer.tsx
• Modified: apps/frontend/hooks/useToast.ts
• Modified: apps/frontend/components/escrow/CreateEscrowWizard.tsx
• Modified: apps/frontend/components/escrow/detail/EscrowHeader.tsx
• Modified: apps/frontend/components/escrow/detail/file-dispute-modal.tsx
• Modified: apps/frontend/components/escrow/detail/ArbitratorResolutionModal.tsx
• Modified: apps/frontend/components/settings/ApiKeyManager.tsx
• Modified: apps/frontend/app/settings/page.tsx
• Modified: apps/frontend/lib/escrow-api.ts

Acceptance Criteria Met

• ✅ Escrow template system complete
• ✅ Toast system with queue/deduplication
• ✅ Countdown timer with expiry warnings and refund button
• ✅ All alert() calls replaced with toasts
• ✅ Accessibility features added
• ✅ Unit tests for countdown logic
  Closes feat(frontend): Build escrow creation from template/preset #421
  Closes feat(frontend): Add toast notification system with queue management #395
  Closes feat(frontend): Implement deadline countdown timer with expiry warnings #410

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