Expand data visualization tools in chat

## Summary
Expand chat visualization beyond existing Chart.js inline charts.

## 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
Start with a gap list: supported chart types, table-to-chart prompts, export fidelity, interaction needs, maps, timelines, diagrams, and whether any new local library is justified.

## 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: Data Visualization Tools In Chat
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension
- Current anchors: `docs/explanation/features/CHAT_BACKEND_INLINE_CHART_HANDOFF.md`, `docs/explanation/features/INLINE_CHART_COLOR_EDITOR.md`, `application/single_app/semantic_kernel_plugins/chart_plugin.py`, `application/single_app/static/js/chat/chat-inline-charts.js`
- Proposed roadmap priority: P2
- Proposed roadmap size: L
- Related: #718 tracks Mermaid diagrams; avoid duplicating that specific scope unless this issue depends on it.