fix: make press nav anchor work with fixed footer

- Move id="presse" from footer to empty div at end of main
- Add scroll-margin-top: 100vh to force max scroll and reveal footer

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

index.html

@@ -474,8 +474,9 @@
           alt="Couverture du livre Le courage du réel"
         />
       </section>
+      <div id="presse"></div>
     </main>
-    <footer id="presse" class="blue-container">
+    <footer class="blue-container">
       <header>
         <h2 class="section-title">Contact</h2>
         <h3 class="title">Presse et médias</h3>