Add self-contained single HTML export

[Mine77]

Summary

• Add starry-slides export html [deck] --out <file> for command-line single HTML export.
• Inline deck-local CSS, CSS url(...), images, audio/video media, posters, and source assets into data: URLs during HTML export.
• Keep editor Single HTML export on the same src/node/html-export.ts runtime path and document the self-contained export contract.

Test Plan

• pnpm test src/node/html-export.test.ts packages/slides-core/src/html-export.test.ts
• NODE_NO_WARNINGS=1 pnpm test src/cli/index.test.ts -t "export html|help variants"
• pnpm exec biome check src/node/html-export-assets.ts src/node/html-export.ts src/node/html-export.test.ts src/cli/index.ts src/cli/index.test.ts src/cli/commands/export/action.ts src/cli/commands/export/index.ts packages/slides-core/src/html-export.ts
• pnpm build
• git diff --cached --check

Notes

Full pnpm lint is currently blocked by pre-existing/unrelated formatting findings in tracked package/test-slide files plus the untracked local starry-slides-demo-day/ deck used for manual export smoke testing. This PR's touched files pass targeted Biome checks.

Manual smoke check performed by exporting starry-slides-demo-day to a single HTML file and opening it via file://; exported slide images loaded from data: URLs with no page errors or failed requests.