feat: pure-CSS hamburger menu for public nav (≤640px)

app/public/assets/css/common.css

@@ -125,6 +125,147 @@ header {
     border-bottom: 1px solid var(--header-nav-active-border);
     padding-bottom: 1px;
   }
+
+  /* nav-top-row: transparent wrapper at desktop — children become
+     direct flex items of nav, preserving the existing layout */
+  .nav-top-row {
+    display: contents;
+  }
+
+  /* nav-mobile-links: mobile-only dropdown, hidden at desktop */
+  .nav-mobile-links {
+    display: none;
+  }
+}
+
+/* ============================================================
+   HAMBURGER MENU — public nav (pure CSS, checkbox trick)
+
+   DOM order inside <header> (public only):
+     input.menu-btn           ← off-screen checkbox
+     nav
+       div.nav-top-row        ← always-visible row (logo + burger)
+         div.nav-left         ← logo + desktop link list
+         ul.nav-right-links   ← desktop right links
+         label.menu-icon      ← burger icon trigger
+       ul.nav-mobile-links    ← full dropdown (hidden by default)
+
+   At desktop: .menu-icon and .nav-mobile-links are display:none.
+               .nav-top-row is display:contents so its children
+               participate directly in nav’s flex row.
+   At mobile:  nav becomes a flex column. .nav-top-row is a real
+               flex row (logo | burger). .nav-mobile-links expands
+               via max-height on checkbox:checked.
+   ============================================================ */
+
+/* Off-screen checkbox — triggered by its label */
+.menu-btn {
+  position: absolute;
+  top: -9999px;
+  left: -9999px;
+}
+
+/* Burger label — takes no space at desktop */
+.menu-icon {
+  display: none;
+  cursor: pointer;
+  padding: var(--space-2xs) var(--space-s);
+  align-items: center;
+  justify-content: center;
+}
+
+/* Middle bar of the burger icon */
+.navicon {
+  background: var(--accent-foreground);
+  display: block;
+  height: 2px;
+  width: 24px;
+  position: relative;
+  transition: all 0.3s ease-out;
+}
+
+/* Top and bottom bars */
+.navicon::before,
+.navicon::after {
+  content: '';
+  background: var(--accent-foreground);
+  display: block;
+  height: 2px;
+  width: 100%;
+  position: absolute;
+  transition: all 0.3s ease-out;
+}
+
+.navicon::before { top: -7px; }
+.navicon::after  { bottom: -7px; }
+
+/* ---- Mobile ---- */
+@media screen and (max-width: 640px) {
+  /* Nav becomes a flex column: top-row on row 1, dropdown on row 2 */
+  header nav[aria-label="Navigation principale"] {
+    display: flex;
+    flex-direction: column;
+    padding: 0;
+  }
+
+  /* Top row: logo left, hamburger right */
+  header nav[aria-label="Navigation principale"] .nav-top-row {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: var(--space-s);
+  }
+
+  /* Hide desktop link lists inside the top row */
+  header nav[aria-label="Navigation principale"] .nav-left-links,
+  header nav[aria-label="Navigation principale"] .nav-right-links {
+    display: none;
+  }
+
+  /* Reveal the hamburger icon */
+  .menu-icon {
+    display: flex;
+  }
+
+  /* Dropdown: clipped by default */
+  header nav[aria-label="Navigation principale"] .nav-mobile-links {
+    list-style: none;
+    margin: 0;
+    padding: 0;
+    max-height: 0;
+    overflow: hidden;
+    transition: max-height 0.2s ease-out;
+  }
+
+  /* ---- Open state ---- */
+  .menu-btn:checked ~ nav[aria-label="Navigation principale"] .nav-mobile-links {
+    max-height: 300px;
+  }
+
+  /* Dropdown link rows */
+  header nav[aria-label="Navigation principale"] .nav-mobile-links li {
+    border-top: 1px solid var(--header-nav-active-border);
+  }
+
+  header nav[aria-label="Navigation principale"] .nav-mobile-links li a {
+    display: block;
+    padding: var(--space-xs) var(--space-s);
+  }
+
+  /* ---- Animate burger → X ---- */
+  .menu-btn:checked ~ nav[aria-label="Navigation principale"] .menu-icon .navicon {
+    background: transparent;
+  }
+
+  .menu-btn:checked ~ nav[aria-label="Navigation principale"] .menu-icon .navicon::before {
+    transform: rotate(-45deg);
+    top: 0;
+  }
+
+  .menu-btn:checked ~ nav[aria-label="Navigation principale"] .menu-icon .navicon::after {
+    transform: rotate(45deg);
+    bottom: 0;
+  }
 }
 
 main {