fix: _print output respects relativeURLs=true (#2638)

[greekera1000]

Fixes #2638

Problem

The _print/ output used absolute (root-relative) URLs even when the site sets relativeURLs = true, while the rest of the site emitted relative URLs. This broke offline/local viewing of printable pages — CSS, JS, and images 404 when _print/* is opened from the file system — and consequently broke local HTML-to-PDF conversion.

Root cause

Hugo only runs its relativeURLs search-and-replace pass on output formats it classifies as HTML (isHTML: true). The print output format left isHTML unset as a workaround for gohugoio/hugo#14381 (relative alias generation), so print pages never went through that rewrite and kept absolute URLs.

Fix

Enable isHTML: true on the print output format (removes the stale FIXME).

Why the #14381 workaround is no longer needed

The print format sets permalinkable: false, and Hugo only generates the aliases that #14381 concerned when both isHTML and permalinkable are true — so enabling isHTML never exercises that path here. The upstream fix for #14381 has also shipped and predates the theme's minimum Hugo (0.160.1).

Testing

Adds an offline fixture-site regression test (tests/fixture-site/print-relative-urls.test.mjs): builds a section with print output enabled and relativeURLs: true, then asserts the _print page emits no absolute (leading-slash) asset URLs and references the site CSS relatively. Red without the fix, green with it.

Also verified manually with a sub-path baseURL (print CSS/JS and processed-image URLs flip from absolute to relative) and in a multilingual build (defaultContentLanguageInSubdir: true), with no spurious _print aliases generated.