Fix Conics overview in dark mode

[japbcoelho]

Summary

• give every page of the editable Conics overview an explicit white fill
• regenerate the README SVG so it remains legible on GitHub's dark theme
• preserve the existing raster output exactly; the regenerated PNG is byte-identical

Verification

• ./scripts/validate.sh — 72/72 tests passed
• SVG and editable Ipe source passed XML validation
• SVG rendered over a GitHub-like dark background with an opaque white canvas
• regenerated PNG SHA-256 is identical to the published PNG
• git diff --check and staged Gitleaks scan passed