Add retina icon assets

[cobalt-arc-dev]

Fixes #30.

Summary:

• Add @2x PNG variants for the toolbar, runner, close, status, and favicon icons.
• Use high-DPI media queries to swap CSS background images on retina displays while preserving the current CSS pixel sizes.
• Add a 32x32 favicon variant and correct the favicon path to the existing site/images directory.

Validation:

• Verified every @2x PNG is present and exactly double the source dimensions.
• Verified all local PNG references in the changed CSS/HTML files resolve.
• Checked CSS brace balance for site/css/style.css and site/css/runner.css.
• git diff --check