Add Quick Look poster for HTML exports

[Mine77]

Summary

• Adds a static Quick Look logo poster to exported single HTML files so macOS Get Info previews show Starry branding instead of the generic HTML preview.
• Keeps the browser presenter experience unchanged by hiding the poster as soon as the runtime script runs.
• Documents the Quick Look behavior in the CLI usage guide and ADR.

Test Plan

• pnpm test packages/slides-core/src/html-export.test.ts src/node/html-export.test.ts src/node/macos-file-icon.test.ts
• pnpm exec biome check packages/slides-core/src/html-export.ts packages/slides-core/src/html-export.test.ts docs/skills-references/STARRY-SLIDES-CLI-USAGE.md docs/adr/0021-adopt-reusable-present-mode-and-single-html-export.md
• pnpm build
• node dist/cli/index.js export html starry-slides-demo-day --out starry-slides-demo-day/.starry-slides/export/quicklook-logo-pr-check.html
• qlmanage -t -s 512 -o /tmp/starry-ql-pr-check /Users/haichao/code/starry-slides/starry-slides-demo-day/.starry-slides/export/quicklook-logo-pr-check.html