Improve token tracker charts and time-window exports

## Summary
Improve token analytics with group token tracker charts and exports that honor selected time windows.

## 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
Include personal/group/public scopes, current date filters, CSV export columns, and chart labels that match visible filters.

## 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: Token Tracker Improvements
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Extension
- Current anchors: `application/single_app/route_backend_control_center.py`, `application/single_app/static/js/control-center.js`, `docs/explanation/features/v0.241.111/STATS_TIME_WINDOWS_AND_EXPORTS.md`
- Proposed roadmap priority: P2
- Proposed roadmap size: M
- Current note: Group token totals are already attached to Control Center group payloads. The remaining work is charting, list integration, and export/time-window consistency.
- Related: #622 tracks Control Center token filtering and #933 tracks token usage limits/notifications. This issue focuses on tracker charts and time-window exports.