Use a web worker for markdown rendering

[jooj211]

Closes #72.

IssueHunt bounty: https://oss.issuehunt.io/r/BoostIO/BoostNote-App/issues/72

Summary

• move markdown parsing, shortcode expansion, sanitizing, and KaTeX processing into a dedicated markdown render worker
• keep CodeMirror highlighting, Mermaid rendering, and the existing React markdown components on the main thread
• preserve the existing main-thread renderer as a fallback if worker creation or worker rendering fails

Verification

• npm run lint
• ./node_modules/.bin/tsc --noEmit --pretty false
• NODE_OPTIONS=--openssl-legacy-provider TS_NODE_PROJECT=tsconfig-webpack.json NODE_ENV=production BOOST_HUB_BASE_URL=http://localhost SSE_URL=http://localhost REALTIME_URL=http://localhost GA_TRACKING_ID=dev GITHUB_OAUTH_ID=dev GOOGLE_CLIENT_ID=dev INTERCOM_APP_ID=dev STRIPE_PUBLISHABLE_KEY=dev COUPONS_NEW_USER_STANDARD=dev COUPONS_NEW_USER_PRO=dev COUPONS_NEW_SPACE=dev COUPONS_NEW_SPACE_ANNUAL=dev MOCK_BACKEND=false BOOST_PDF_EXPORT_BASE_URL=http://localhost GITHUB_APP_URL=http://localhost LEGACY_CUTOFF=0 ./node_modules/.bin/webpack --mode production --config webpack.cloud.config.ts

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