Add width/height attributes to all img elements to prevent CLS

Problem

All <img> elements across the site are missing explicit width and height attributes. Without intrinsic dimensions, the browser cannot reserve layout space before images load, causing layout shift (CLS).

Current CLS is estimated borderline (0.05–0.15, threshold is ≤0.1). This is the primary CLS risk.

Affected images

• Hero WebP in layouts/partials/hero.html — Hugo-processed, dimensions available via .Width/.Height
• Logo PNG in layouts/partials/nav.html and layouts/partials/footer.html — displayed at 32×32
• Dublin gallery images in layouts/partials/discover-dublin.html — Unsplash URLs with w= and h= params
• About section images in layouts/partials/about.html — Unsplash URLs
• Speaker photos (when speakers are added)

Also: the featured gallery image in discover-dublin.html is missing loading="lazy" (it is below the fold but eagerly loaded).

Fix for Hugo-processed images

```html

```

Fix for Unsplash images

Add explicit dimensions matching the w= and h= query parameters already in the URLs.

Start by reviewing the image elements in layouts/partials/hero.html, nav.html, footer.html, discover-dublin.html, and about.html. Use Hugo image dimensions for the hero and match the existing Unsplash w= and h= parameters elsewhere; add lazy loading to the below-the-fold featured gallery image. Done means every listed image has explicit dimensions and the gallery image is lazy-loaded.