feat: creator certification, portfolio optimization, social proof and accountability score (#852-#855)

[MJigah]

Implements the four open feature issues assigned to MJigah, one commit each. Following the existing pattern (pure src/lib module + vitest file + Tailwind component + component test); components are self-contained and not yet mounted on a page.

• Implement advanced creator certification system #852 Creator certification – lib/creatorCertification.ts, components/creator/CreatorCertification.tsx. Bronze/silver/gold tiers with sequential requirements, next-tier checklist and progress bar.
• Add app invoice portfolio optimization recommendations #853 Portfolio optimization recommendations – lib/portfolioOptimization.ts, components/recommendations/PortfolioOptimization.tsx. Flags overdue/expiring/stale open invoices, recipient concentration, batchable small invoices and high cancellation rates, most urgent first.
• Add app social proof widgets — show recent payments #854 Social proof widgets – lib/socialProof.ts, components/pay/SocialProofWidget.tsx. Recent payments with masked payer addresses, relative times and a 24h payer count; renders nothing when empty.
• Implement creator accountability score #855 Creator accountability score – lib/creatorAccountability.ts, components/creator/CreatorAccountabilityScore.tsx. 0-100 score (fulfilment 50, disputes 30, responsiveness 20), letter grade, and an "unrated" state under 3 invoices.

Layouts use responsive Tailwind classes and ARIA roles (progressbar, meter, aria-live).

Tests, type-checking and the build were not run for this change, so the acceptance criteria "all tests pass", "TypeScript compiles" and "npm run build succeeds" are unverified; CI will be the first run.

Closes #852
Closes #853
Closes #854
Closes #855

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