chore(web): migrate to Next 16 native lint config

[yashranaway]

Closes #81.

What changed

• Upgrade Next and eslint-config-next to 16.3.
• Replace FlatCompat with the native flat configurations for Core Web Vitals and TypeScript.
• Remove the legacy @eslint/eslintrc and @rushstack/eslint-patch path.
• Fix the two React Hooks findings exposed by the native configuration.
• Adopt the generated Next 16 agent guidance and stop tracking generated next-env.d.ts.
• Keep ESLint 9 temporarily: the React/import/a11y plugins bundled by Next 16 fail on ESLint 10. Follow-up Upgrade web lint to ESLint 10 after the Next plugin stack supports it #93 tracks removing that hold, and Dependabot now suppresses the known-failing major bump.

Verification

• pnpm install --frozen-lockfile --filter @headless/web
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• Rendered Playwright smoke test at desktop and mobile viewports
  • page identity and meaningful content
  • no Next error overlay or console errors
  • animated scan frame visible
  • theme toggle updates DOM state, accessible label, and local storage
  • no mobile horizontal overflow

The existing THREE.Clock and headless WebGL performance warnings remain tracked by #51.