Complete roadmap recovery and UX gaps

[Kalmuraee]

Summary

• add stream interruption recovery with partial response preservation and retry using saved prompt/context
• add quick model/cost UX, side-by-side diff preview, rollback redo support, and compressed fallback screenshots
• add proxy and WebSocket protocol regression tests, and test the production LLM JSON extractor directly

Verification

• npm run typecheck
• npm run lint
• npm test
• npm run check:all
• npm run prepublishOnly && node --check dist/toolbar/index.global.js