Add a first-party Manifest V3 browser extension

[Agent-QG]

Summary

• add a first-party Manifest V3 extension under browser-extension/
• export comments, replies, resolved state, tracked changes, Markdown, JSON, JSONL, and a response-letter scaffold from the current Overleaf tab
• add English/Chinese UI and report output, UTC-stamped export folders, and per-comment/change timestamps
• document installation, privacy, permissions, and the relationship with the Python app
• add an isolated Node test workflow for extension changes

Safety and privacy

The extension stays within the project's existing read-only boundary. It requests only activeTab, scripting, and downloads; it does not request cookies, run a backend, upload project content, post replies, or resolve threads.

Verification

• npm run check
• npm test — 6 passed
• pytest — 182 passed, 9 skipped
• Chrome unpacked-extension packaging smoke test

Closes #5