Canvas mode: export annotations as SVG/vector package

[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 produces an SVG/vector package for the current canvas composition.

Scope

This first version should vectorize the annotation layer, not perform true raster image tracing.

• Embed the source image as an SVG <image> element.
• Render Canvas annotations as SVG elements:
  • pen/freehand/arrow paths -> <path>
  • boxes -> <rect>
  • memos -> <text> or grouped text elements
• Use the source image natural dimensions as the SVG viewport.
• Map normalized annotation coordinates into SVG pixel units.
• Export should not mutate canvas state or create a canvas version file.

Out of scope

• True AI/raster vectorization of the generated image.
• Potrace-like bitmap tracing.
• Editable SVG import back into Canvas Mode.
• Server-side file writes.

Acceptance criteria

• Canvas toolbar exposes an SVG export action.
• SVG contains the source image plus vector annotation overlays.
• Pen, arrow, box, and memo positions match PNG export visually.
• SVG export works for unsaved local annotations.
• Export path does not expose raw local filesystem paths.
• Tests cover SVG dimensions, embedded image behavior, annotation element generation, and no state mutation.

Notes

If true raster vectorization is desired later, track it as a separate issue. Generated raster images are unlikely to vectorize cleanly without a dedicated tracing pipeline and UX warnings.