feat: add navigation anchors and smooth scroll

- Link all navigation items to their corresponding sections
- Add smooth scroll behavior on html element
- Add scroll-margin-top on sections to compensate for fixed header
- Add id="presse" to footer for press navigation link

Navigation mapping:
- Le Courage du Réel → #hero
- Extraits → #excerpts
- Les poignées → #handles
- Le système → #system
- L'auteur → #author
- Précommander → #buy
- Presse → #presse (footer)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

assets/css/src/_reset.scss

@@ -1,3 +1,7 @@
+html {
+  scroll-behavior: smooth;
+}
+
 body,
 html,
 ul,
@@ -45,3 +49,7 @@ main {
   background-color: #fff;
   margin-bottom: 50rem;
 }
+
+section {
+  scroll-margin-top: calc(4.8125rem + 4rem);
+}

assets/css/style.css

@@ -1,3 +1,7 @@
+html {
+  scroll-behavior: smooth;
+}
+
 body,
 html,
 ul,
@@ -46,6 +50,10 @@ main {
   margin-bottom: 50rem;
 }
 
+section {
+  scroll-margin-top: 8.8125rem;
+}
+
 :root {
   --font-narrow: 'owners-xnarrow', sans-serif;
   --font-size-l: 4rem;