Update deps and setup of giftcard-tutorial project

[pfferrari]

Closes commercelayer/issues-app/issues/716

Summary

Resolved all pnpm audit vulnerabilities in solutions/giftcard-tutorial (4 high, 2 moderate) by upgrading Next.js 15 → 16.

Security fix

• All 6 advisories were transitive dependencies of next@15.5.25, which pins postcss@8.4.31 (exact) and sharp@0.34.5.
• No 15.x release patches them; next@16.3.4 is the first version pinning postcss@8.5.23 and sharp@^0.35.4.
• Aligns the project with cms/nextjs-contentful-store and webhooks/nextjs-signature-verification, already on Next 16.

Dependency changes

• next → ^16.3.4, eslint-config-next → ^16.3.4.
• Removed @eslint/eslintrc (only needed for FlatCompat).

Next.js 16 breaking changes addressed

• next.config.ts: images.domains (deprecated) → images.remotePatterns.
• package.json: lint script next lint → eslint . (next lint was removed in Next 16).
• eslint.config.mjs: migrated to the native flat config from eslint-config-next v16.
• tsconfig.json / next-env.d.ts: mandatory changes applied by Next 16 (jsx: react-jsx, .next/dev/types).
• Added AGENTS.md / CLAUDE.md, matching the other Next 16 projects, so next dev doesn't recreate them as uncommitted changes.

Configuration

• Added .env.example documenting NEXT_PUBLIC_CL_CLIENT_ID and NEXT_PUBLIC_CL_ORG_SLUG.
• Added optional NEXT_PUBLIC_CL_MARKET_CODE, falling back to us (the previously hardcodardcoded market:code:us scope in src/lib/commercelayer.ts.

Verification

• pnpm audit — no known vulnerabilities.
• pnpm lint — clean.
• tsc --noEmit — clean.
• pnpm build — passes with real credentials (4/4 static pages generated).
• Not verified: the interactive browser flow (amount/design selection → gift card creation

Checklist (general changes)

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes.
• You are NOT deprecating/removing a feature.

[netlify]

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