Optimize page images and remove unused assets

[skratchdot]

Reduce image downloads flagged by mobile Lighthouse: replace the 48.7 KiB chart texture with a 4.4 KiB WebP and the 97.3 KiB ICO with a 1.2 KiB, 32×32 PNG shared by the favicon and footer. Give the footer image explicit dimensions and preload the chart texture on the home page so the browser discovers it before the playground mounts.

Remove the replaced image files and two unused legacy Chosen sprites. Keep the README preview image and GitHub Pages .nojekyll file.

Validation:

• pnpm run check: passed, including 362 unit tests and production build.
• Three browser checks passed: public asset loading, responsive header layout, and chart resizing.
• Rebuilt after deleting obsolete assets; generated output has no references to them.
• Compared original and compressed texture visually; formatting and whitespace checks passed.
• Mobile Lighthouse on the compressed local build passes image delivery and LCP resource discovery checks. Its performance score was 85; the live baseline scored 83 in a separate run. These environments are not directly comparable, and this does not establish a deployed score improvement. JavaScript execution remains a performance bottleneck.