Load Mermaid/Highlight.js only where used; fix heading hierarchy and image attributes - #882

Closes #865, #860, #869, #861. Related: #870 (duplicate preconnects, item 6).

What changes

Conditional Mermaid / Highlight.js (#865)

• diagram and highlight site params default to false. site_head.html decides per page: front matter (highlight: true / diagram: true) wins; otherwise the library loads only when the page's Markdown content contains a code block (resp. a Mermaid block). Detection runs on .Content, so the cite modal's <pre><code class="tex hljs"> on every page does not trigger it.
• Both libraries load with defer; their only callers sit in academic.js's document-ready handler.
• Result on a full build: Highlight.js loads on one page (/manuscript/, forced via front matter because its code block lives in a headless section file); Mermaid loads on none; the homepage requests neither.

Search index (#861)

• Lazy loading of /index.json landed in Fix navbar search: restore missing fields so results, links, and full-text match  #871. This PR adds the missing failure state: if the fetch fails, the overlay shows a message and the next search-open retries.
• Verified locally: no index.json request on a cold homepage load; one request after opening search; a direct /?q=preregistration URL renders results.

Heading hierarchy (#860)

• Search overlay heading is an h2 (same visual size via CSS).
• Page-builder pages get exactly one h1: a hero widget renders a visible one, otherwise widget_page.html emits a visually hidden h1 from the page title (site title on the homepage). All section titles from widget partials are h2 with unchanged size and divider.
• Author profiles: the person's name is the h1.
• Site-wide, pages with more than one h1 drop from 5287 to 13. The remaining 13 are content-level # headings inside widget sections (community, mission, testimonials, publications, …) and shortcode-emitted h1s; those are content edits and left out here.

Images (#869)

• Every template <img> now has an alt decision. The award logos in content/awards/awards.md get alt="" because the adjacent text names the awarding organisation; the ELD CAMA diagram gets a descriptive alt.
• Intrinsic width/height added to announcement cards, list cards, resource cards, avatars, page-header featured images, the CC badge, and featurette images (via a new functions/static_img_dims partial that reads static raster files at build time). CSS keeps those images fluid (height: auto). Homepage: 15 of 24 images now carry dimensions; the rest are the SVG logo, SDG badges in a shortcode, the chatbot icon and one content image.

Cleanup

• Duplicate Google Fonts preconnects removed from chatbot.html.

Regression check

scripts/check_seo_html.py public parses every built page and fails on: an <h1>Search</h1>, Mermaid/Highlight.js on the homepage, a library loaded on a page without a matching code block (or vice versa), or an <img> without an alt attribute. It also reports pages with zero or several h1s and the homepage image dimension count. It runs after the Hugo build in deploy.yaml.

Verified

• Full local build passes all four checks.
• Screenshots compared against a baseline build: homepage sections, search overlay, author profile and featurette images look the same.

Not in this PR

#859, #864, #866, #867, #868 need expert review or design decisions.

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