feat: add Earnings & Referrals dashboard layout with responsive structure and loading skeletons (Closes #1154)

[waterWang]

Summary

This PR implements Closes #1154 — Build Referral Dashboard Layout for the affiliates area.

What's included

1. New component apps/web/components/affiliates/earnings-referrals-dashboard.tsx

   • Earnings Summary section — 4 KPI cards (Total Earnings, Pending Payouts, Withdrawable, Lifetime Clicks) using the existing AnalyticsKpiCard component from the design system
   • Referral Performance section — table with event name, clicks, sales, commission, and creation date; handles empty state natively
   • Payout History section — placeholder with "Coming soon" badge, ready for backend integration
   • Full loading skeleton — separate skeletons for each section (earnings summary, referral table, payout history) for a polished loading experience
   • Empty state — displayed when the affiliate has no activity yet, with a prompt to generate referral links
   • Error state — graceful fallback when API calls fail
   • Responsive layout — grid-cols-1 sm:grid-cols-2 lg:grid-cols-4 for earnings cards, table scrolls horizontally on mobile

2. Updated apps/web/app/affiliates/dashboard/page.tsx

   • Replaced the old AffiliateMetricsDashboard with the new EarningsReferralsDashboard
   • Updated page title from "Performance Metrics" to "Earnings & Referrals"
   • Retained the ReferralLinkGenerator component for link creation

Design decisions

• Uses the existing project design system (border-2 border-black, shadow-[-6px_6px_0_rgba(0,0,0,1)], bg-surface, text-ink-deep, etc.)
• Reuses AnalyticsKpiCard from analytics-kpi-card.tsx for consistent metric display
• API call gracefully falls back to placeholder data when the backend (## BACKEND: Create Affiliate Registration Endpoint #1150) is not yet deployed
• All sections are self-contained and ready for real data integration once the affiliate registration endpoint is live

Testing

• Component renders without errors
• Loading skeleton displays correctly
• Empty state displays when no activity data
• Error state displays when API call fails
• Layout is responsive across breakpoints
• Follows existing project coding conventions

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