Add image editing and reference-image generation

## Summary
Add image editing and image-to-image/reference-image generation using uploaded images or selected workspace files.

## 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
Handle image selection, permissions, source attribution, model/provider capability detection, prompt editing, and storage of generated results.

## 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: Image Editing And Reference Image Generation
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: New / Extension
- Current anchors: `docs/explanation/features/IMAGE_PROPOSAL_PIPELINE.md`, `application/single_app/functions_image_generation.py`, `application/single_app/static/js/chat/chat-inline-image-proposals.js`
- Proposed roadmap priority: P2
- Proposed roadmap size: L