Reuse image popup for chat and workspace images

## Summary
Reuse the New Features/Admin image popup pattern for viewing generated, uploaded, and workspace images in chat.

## User Value
Tracking this as a GitHub issue moves the backlog item out of a private markdown file and gives the team a clear place to prioritize, design, implement, and validate the work.

## Proposed Behavior
Support keyboard controls, mobile sizing, blob-backed authorization, and images embedded in exports or proposal cards.

## Acceptance Criteria
- [ ] The requested behavior is designed or implemented for the relevant SimpleChat surfaces.
- [ ] Permission, configuration, and existing-feature interactions are accounted for.
- [ ] Tests, documentation, or validation notes are added as appropriate.

## Notes
- Source backlog entry: Reuse Image Popup For Chat And Workspace Images
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension
- Current anchors: image proposal cards, enhanced citations/image previews, workspace document viewers
- Proposed roadmap priority: P2
- Proposed roadmap size: M