Fix light mode nav invisible, logo § calligraphié, typo ronde fine

- Navigation : btn-ghost, active-class, footer, nav mobile → CSS vars adaptatifs
- Overrides light mode renforcés avec !important (scoped styles compatibles)
- btn-primary garde text-white en light mode (fond coloré)
- Logo : symbole § calligraphié SVG inline remplace lucide-book-open
- Logo text : font-display weight 300 (ronde fine) + text-gradient
- Logo SVG aussi exporté en /public/images/logo-section.svg
- Header/footer : backgrounds et bordures via CSS vars

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

app/components/layout/NavMobile.vue

@@ -11,7 +11,7 @@
     <Transition name="slide-menu">
       <nav
         v-if="open"
-        class="fixed inset-y-0 right-0 z-50 w-72 bg-surface-600 border-l border-white/8 p-6 shadow-2xl"
+        class="fixed inset-y-0 right-0 z-50 w-72 bg-[hsl(var(--color-surface))] border-l border-[hsl(var(--color-text)/0.1)] p-6 shadow-2xl"
         aria-label="Menu mobile"
       >
         <div class="flex items-center justify-between mb-8">
@@ -29,7 +29,7 @@
           <li v-for="item in nav" :key="item.to">
             <NuxtLink
               :to="item.to"
-              class="flex items-center gap-3 rounded-lg px-4 py-3 text-base font-medium transition-colors hover:bg-white/5"
+              class="flex items-center gap-3 rounded-lg px-4 py-3 text-base font-medium text-[hsl(var(--color-text)/0.7)] transition-colors hover:bg-[hsl(var(--color-text)/0.06)] hover:text-[hsl(var(--color-text))]"
               active-class="bg-primary/10 text-primary"
               @click="emit('update:open', false)"
             >