a11y: nav aria-label, search role=search + label, card hover motion guard

- templates/nav.php: add aria-label="Navigation principale" to <nav>; emit
  aria-current="page" on the active link alongside the existing CSS class
  so screen readers announce the current page without relying on colour/style alone

- templates/search-bar.php: add role="search" + aria-label="Recherche" to
  the <form>; add a visually-hidden <label for="site-search-input"> linked to
  the input via id="site-search-input", satisfying WCAG 3.3.2 (labels/instructions)
  and 4.1.2 (name/role/value) — placeholder text alone is not a label

- public/assets/main.css: add @media (prefers-reduced-motion: reduce) block that
  sets transition:none and transform:none on .card__media img/video hover, so the
  scale(1.02) zoom is fully suppressed for users who opt out of motion (WCAG 2.3.3 /
  prefers-reduced-motion); the global transition-duration guard in common.css already
  covers all other transitions but does not zero the transform value itself

Fixes TODO sections: G (nav/search-bar landmark names), I (site-search form ARIA),
3.3.2 (search input label), prefers-reduced-motion (card hover transform gate)

public/assets/main.css

@@ -215,3 +215,15 @@
   font-weight: 600;
   color: var(--black);
 }
+
+/* Suppress card hover scale for users who prefer reduced motion */
+@media (prefers-reduced-motion: reduce) {
+  .card__media img,
+  .card__media video {
+    transition: none;
+  }
+  .card:hover .card__media img,
+  .card:hover .card__media video {
+    transform: none;
+  }
+}