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/partials/nav.html

@@ -10,7 +10,7 @@
     :class="$store.nav.open || scrolled ? 'bg-ink/95 backdrop-blur-md shadow-[0_1px_0_var(--color-zinc)]' : ''"
   >
     <!-- Logotype -->
-    <a href="/" class="logo-glitch flex items-center gap-3 hover:opacity-90 transition-opacity" aria-label="{{ .Site.Title }} Home">
+    <a href="/" class="logo-glitch flex items-center gap-3 hover:opacity-90 transition-opacity" aria-label="{{ .Site.Params.logoText }} Home">
       {{- partial "logo.html" (dict "id" "nav" "class" "h-8 w-auto flex-shrink-0") -}}
       <span class="text-gradient font-display text-2xl md:text-3xl leading-none tracking-wide">
         {{- .Site.Params.logoText -}}