fix(desktop): clean artifacts timestamps and browser assets

[tt-a1i]

Fixes #48350

Summary

• Normalize artifact timestamps stored as epoch seconds before sorting and display, including message and session fallback timestamps.
• Filter browser/CDP/Playwright tool results so page asset URLs from snapshots are not indexed as user artifacts.
• Preserve explicit browser artifact fields such as screenshot_path, including JSON payloads wrapped in <untrusted_tool_result>.

Tests

• npm run test:ui --workspace apps/desktop -- src/app/artifacts/index.test.ts
• npm run typecheck --workspace apps/desktop
• cd apps/desktop && npx eslint src/app/artifacts/index.tsx src/app/artifacts/index.test.ts
• npx prettier --check apps/desktop/src/app/artifacts/index.tsx apps/desktop/src/app/artifacts/index.test.ts
• git diff --check origin/main...HEAD

Notes

• Full npm run test:ui --workspace apps/desktop was attempted earlier and fails in this checkout on existing unrelated localStorage/platform baseline failures outside this diff; the focused artifacts test passes.