Summary

• add a browser-side notebook-to-markdown serializer for Drive search sidecars
• switch LocalNotebooks markdown sync off of ParserService.Serialize
• cover the serializer and sidecar upload path with focused tests

Testing

• pnpm --dir app exec tsc --noEmit --target es2022 --module esnext --moduleResolution bundler --jsx react-jsx --lib es2022,dom --types vitest/globals,node --skipLibCheck src/lib/markdown/serializeNotebookToMarkdown.ts src/lib/markdown/serializeNotebookToMarkdown.test.ts
• local pnpm --dir app test --run ... is blocked on this machine by a Rollup native-module code-signing issue; rely on GitHub Actions for the authoritative test run