Add SharePoint Online and on-prem SharePoint sync

Summary

Add SharePoint source support, likely split into SharePoint Online via Microsoft Graph and SharePoint on-prem via a separate auth/network path.

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

Decide whether SharePoint content is synced into workspaces, searched remotely, or both. Include path browsing, delta/change tracking, permissions, throttling, and private network requirements.

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: SharePoint Online And SharePoint On-Prem Sync
• Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
• Backlog status: New / Extension
• Current anchors: File Sync provider abstractions in application/single_app/functions_file_sync.py; source type constants include sharepoint_on_prem but it is not implemented.
• Proposed roadmap priority: P2
• Proposed roadmap size: XL