fix: address Lighthouse SEO and performance audit failures

- nav: fix label-content-name-mismatch — aria-label now uses logoText
  ("PIVOINE") to match the visible link text (WCAG 2.5.3)
- index: improve link-text — "Learn more" → "About the magazine"
  so the /about/ CTA has a descriptive accessible name
- head: load Google Fonts async via media=print/onload pattern to
  eliminate render-blocking resource penalty (~480ms est. savings)

Remaining items needing server-side action:
- is-crawlable: x-robots-tag: none,... header must be removed in the
  web server / CDN config (not controllable from this repo)
- image-delivery: large banner PNGs on live site need manual WebP
  conversion or migration to Hugo page bundles for auto-processing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/index.html

@@ -137,7 +137,7 @@
     <p class="font-display text-3xl md:text-5xl text-paper leading-tight text-balance mb-10">
       {{ .Site.Params.description }}
     </p>
-    <a href="/about/" class="btn btn-outline">Learn more {{ partial "icon.html" "arrow-right" }}</a>
+    <a href="/about/" class="btn btn-outline">About the magazine {{ partial "icon.html" "arrow-right" }}</a>
   </div>
 </section>