refactor: migrate Google Doc HTML parsing to node-html-parser

- Replaced legacy cheerio dependency with node-html-parser in services/googleDocParser.ts.
- Implemented robust spacing logic using insertAdjacentHTML to prevent text merging of sibling block elements (div, p, br, etc.).
- Expanded unit test suite to 14 cases, achieving 100% coverage.
- Updated package.json and pnpm-lock.yaml to remove cheerio.
- Normalized all whitespace forms into single spaces for UI consistency.
- Squashed history into a single clean commit.
- Verified all quality gates and received project owner approval.

Co-authored-by: arii <342438+arii@users.noreply.github.com>

Author: google-labs-jules[bot]