Canvas mode: export current canvas composition to PPTX

[lidge-jun]

Source

Canvas Mode follow-up from devlog/_plan/0.24-canvas-mode/FOLLOW-UP-ISSUES.md and parent issue #22.

Goal

Add a Canvas export option that creates a PowerPoint deck from the current canvas composition.

Recommended MVP

• Create a one-slide PPTX deck from the current Canvas view.
• Preserve source image aspect ratio on a predictable 16:9 slide.
• Include the source image and visible Canvas annotations.
• Convert boxes, arrows, and memos to PowerPoint shapes/text where practical.
• For complex freehand paths, allow a transparent annotation overlay image fallback.

Implementation direction

Prefer browser-side PPTX export for the first PR, likely with a dependency such as pptxgenjs, because Canvas Mode already has the source image element and annotation state in the UI.

Out of scope

• Multi-image deck export.
• Editable PPTX import back into Canvas Mode.
• Full slide theme/template authoring.
• Batch gallery-to-PPTX export.

Acceptance criteria

• Canvas toolbar exposes a PPTX export action.
• Exported deck opens in PowerPoint, Keynote, or LibreOffice.
• One slide contains the current image and visible annotations.
• Memo text is readable and positioned near the Canvas memo location.
• The feature works without saving the canvas version first.
• Tests cover action wiring, dependency/package inclusion, and payload/export contract.
• Manual QA opens the resulting deck and checks annotation placement.