fix: show live tool result images outside event groups

[jmfederico]

Summary

• keep native tool-result images visible outside collapsed EVENTS groups for both hydrated history and live sessions
• reconcile provisional tool.end images with authoritative message.end content without duplication or metadata loss
• support text-plus-image and image-only results while retaining one technical execution representation
• present visible images as clearly labeled standard chat cards using the assistant color scheme
• keep late-loading images pinned only while the reader remains at the bottom

Credit

This builds on #53 and retains @kanazawahere's original commit, grouping fix, tests, and authorship in the branch history.

Validation

• npm run verify: 167 test files, 1124 passed, 2 skipped
• npm run build
• npm run changelog:status: patch bump recognized
• synthetic merge with current origin/main: 185 test files, 1335 passed, 2 skipped, production build passed
• git diff --check

Supersedes #53.