Add inline artifact timeline cards

[heimanba]

Summary

• render assistant-delivered artifacts inline in the run timeline, including images, videos, app/file links, and delivered files
• add a fullscreen lightbox preview for inline media artifacts
• refine composer/settings/model selector layout behavior and add focused regression tests
• backfill provider config fields across providers so settings can switch providers with env-backed values

Validation

• bun run --cwd apps/webui test
• bun run --cwd apps/server test
• bun run --cwd apps/webui typecheck
• bun run --cwd apps/server typecheck
• bun run lint:changed
• pre-push verify:push ran typecheck, architecture, and full tests, then stopped at lint-changed because the computed push BASE was not a local commit; lint:changed passed separately