fix(viewer): respect base paths for asset surfaces

[benvinegar]

Summary

• include configured base paths in REST asset upload URLs
• load native image and trace assets through the viewer host base path
• add API and embedded-viewer regressions for prefixed asset routes

Review

• Ran three subagent code reviews (correctness, tests/coverage, maintainability). Correctness and tests reported no findings. Maintainability findings were addressed by encoding server asset ids and extracting shared embed bundle serving into e2e fixtures.

Tests

• npm run format:check
• npm run lint
• npm run typecheck
• npm test
• npx playwright test e2e/uploads.spec.ts --project=chromium

This PR description was generated by Pi using GPT-5.1 Codex