Canvas mode: alpha-aware transparent canvas controls

[lidge-jun]

Source

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

Goal

Add alpha-aware Canvas Mode controls for transparent PNG workflows.

Scope

• Detect PNG alpha and show a checkerboard preview in Canvas Mode.
• Add explicit export background choices:
  • source image
  • transparent
  • solid color
• Preserve transparent background in PNG export when selected and valid.
• Keep normal opaque exports unchanged.
• Improve mask preview so selected mask/alpha regions are visually distinct from transparent-background preview.

Existing related code

• PNG alpha validation exists in mask/edit paths.
• Reference compression has transparency-aware behavior.
• Canvas export currently produces PNG from the source image plus annotations, but there is no user-facing transparent-canvas mode or background picker.

Out of scope

• Full layer stack/compositor.
• Per-object opacity controls.
• AI background removal.
• SVG/PPTX export.

Acceptance criteria

• PNGs with alpha show a checkerboard preview in Canvas Mode.
• PNG export can preserve transparent background when selected.
• Export with solid background produces an opaque PNG.
• Mask edit preview remains distinct from alpha-background preview.
• Existing reference compression still preserves alpha when required.
• Tests cover PNG alpha detection, export background mode, and no regression in normal opaque exports.