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>

public/images/logo-section.svg

@@ -0,0 +1,53 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 80" fill="none">
+  <!-- Calligraphic § (section sign) - elegant, flowing strokes -->
+  <path
+    d="M38 8
+       C28 6 18 10 18 20
+       C18 28 26 32 34 34
+       C42 36 48 40 48 48
+       C48 52 46 55 42 57
+       L44 40
+       C44 36 40 32 34 30
+       C28 28 22 24 22 18
+       C22 14 24 11 28 10
+       Z"
+    fill="currentColor" opacity="0.85"
+  />
+  <path
+    d="M26 72
+       C36 74 46 70 46 60
+       C46 52 38 48 30 46
+       C22 44 16 40 16 32
+       C16 28 18 25 22 23
+       L20 40
+       C20 44 24 48 30 50
+       C36 52 42 56 42 62
+       C42 66 40 69 36 70
+       Z"
+    fill="currentColor" opacity="0.85"
+  />
+  <!-- Upper curve -->
+  <path
+    d="M20 16
+       C20 8 28 4 36 6
+       C42 8 46 14 44 20"
+    stroke="currentColor" stroke-width="2.5" stroke-linecap="round" fill="none" opacity="0.7"
+  />
+  <!-- Lower curve -->
+  <path
+    d="M44 64
+       C44 72 36 76 28 74
+       C22 72 18 66 20 60"
+    stroke="currentColor" stroke-width="2.5" stroke-linecap="round" fill="none" opacity="0.7"
+  />
+  <!-- Calligraphic flourish - top -->
+  <path
+    d="M36 4 Q42 2 46 6"
+    stroke="currentColor" stroke-width="1.5" stroke-linecap="round" fill="none" opacity="0.5"
+  />
+  <!-- Calligraphic flourish - bottom -->
+  <path
+    d="M28 76 Q22 78 18 74"
+    stroke="currentColor" stroke-width="1.5" stroke-linecap="round" fill="none" opacity="0.5"
+  />
+</svg>