Add reading progress bar for articles

[mortennordbye]

Describe your changes

Adds an optional reading progress bar for articles: a thin, primary-colored bar fixed to the top of the viewport that fills from 0 to 100 percent as the reader scrolls through a page.

Why

Long-form articles have no scroll-position indicator today. This is a common, lightweight blog-theme affordance and there is no equivalent in the theme (the scroll-to-top button only appears past a threshold and shows no progress).

What

• New article.showReadingProgress param (default false), overridable per page via front matter, following the same pattern as the other article.show* flags.
• New layouts/partials/reading-progress.html partial and assets/js/reading-progress.js, mirroring the existing scroll-to-top feature.
• The script is loaded as its own fingerprinted <script> tag (the same approach used for zen-mode.js) rather than appended to the shared main.bundle.js, because that bundle is cached by its target name and the inclusion is page-conditional.
• The bar sits at z-100 so it renders above the fixed header while still being covered by the search overlay (z-500).
• Enabled on the exampleSite to showcase it, and documented in the configuration and front matter tables.

Verification

• hugo production build of the exampleSite succeeds with no errors; the script tag and bar element are emitted on article pages and absent on the home and list pages.
• Verified in the browser that the bar tracks scroll exactly (0 percent at top, 50 percent at mid, 100 percent at bottom) and paints above the fixed header, in both light and dark mode.
• Only the .h-1 utility was added to the committed compiled CSS (via npm run build); no other CSS changed.

Issue ticket number and link

N/A

Checklist before requesting a review

• I have performed a self-review of my code
• Will this be part of a product update? If yes, please write one phrase about this update.

Optional reading progress bar for articles, enabled with article.showReadingProgress.

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