Support Word and PowerPoint templates for generated files

[paullizer]

Summary

Let users provide Word/PowerPoint templates when generating new documents or presentations.

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

Split template selection, template storage, placeholder mapping, model output contract, and fallback behavior. Consider personal/group template permissions.

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: Template-Driven Word And PowerPoint Generation
• Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
• Backlog status: Extension
• Current anchors: docs/explanation/features/v0.241.105/MESSAGE_POWERPOINT_EXPORT.md, SimpleChat native Word/PowerPoint upload operations, conversation export routes
• Proposed roadmap priority: P2
• Proposed roadmap size: L
• Related: generate word, ppt, excel, and csv as outputs #482 covered generating Word, PowerPoint, Excel, and CSV outputs; this issue focuses on user-provided templates.