Improve mobile PageSpeed performance and accessibility

[Copilot]

Addresses mobile PageSpeed findings affecting image delivery, layout stability, accessibility, LCP discovery, and SEO metadata.

Changes

• Image delivery

  • Added WebP variants for homepage images, reducing payloads by ~75–80%.
  • Retained PNG fallbacks.
  • Enabled post cards to use matching WebP assets automatically.

• Loading performance

  • Preload the homepage’s rendered WebP hero only on the homepage.
  • Lazy-load and asynchronously decode non-priority images.

• Layout stability

  • Added intrinsic dimensions to logo, hero, profile, and post-card images.

• Accessibility

  • Added an explicit decorative alt to the header logo.
  • Replaced the unresolved navigation aria-labelledby with aria-label.

• SEO

  • Generate normalized, page-specific meta descriptions capped at 160 characters, with a site-level fallback.