5f8ffd1c

By: Michael Lynch <git@mtlynch.io>

Prefetch adjacent images via JS to fix Firefox CSP compatibility

<link rel="prefetch"> CSP behaviour varies by browser: Chrome 112+ uses
the type-based img-src directive (so it worked), while Firefox falls back
to default-src, which only allows 'self' and therefore blocked cross-origin
S3/CDN prefetch requests. prefetch-src is not implemented in Firefox, so
adding that directive to the CSP has no effect there.

Replace the <link rel="prefetch"> tags with new Image() calls in JS.
Image() loads are governed by img-src in all browsers, which already
includes the media origin. The adjacent-image URLs are rendered as
data attributes on the content element (per the project convention of
not embedding Go template conditionals in JavaScript) and the JS reads
them after DOMContentLoaded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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