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)

templates/search-bar.php

@@ -3,13 +3,16 @@
 // $searchValue: current search query (optional)
 $_sbValue = $searchBarValue ?? $_GET['query'] ?? '';
 ?>
-<form class="site-search" method="GET" action="/search.php">
+<form class="site-search" method="GET" action="/search.php"
+      role="search" aria-label="Recherche">
+    <label for="site-search-input" class="sr-only">Recherche</label>
     <svg class="site-search__icon" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"
          fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"
          aria-hidden="true" focusable="false">
         <circle cx="11" cy="11" r="8"/><line x1="21" y1="21" x2="16.65" y2="16.65"/>
     </svg>
     <input
+        id="site-search-input"
         class="site-search__input"
         type="text"
         name="query"