feat: responsive images for investigation cover cards

- Add `cover-card` and `cover-card-webp` srcsets in config
  Widths [465, 728, 930, 1000, 1456] couvrent 1x et 2x (retina)
  pour les 3 breakpoints : ≥1000px fixe 465px, 728–1000px 50vw, <728px 100vw
- Refactor snippet picture.php : accepte $sizes (string complète),
  $srcsetName (srcset nommé), $alt (override), fallback webp automatique
- Update investigations.php : remplace <img> nu par snippet picture
  avec sizes et srcsetName adaptés

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

site/templates/investigations.php

@@ -42,7 +42,13 @@
 
     <?php if ($cover): ?>
     <figure>
-        <img src="<?= $cover->url() ?>" alt="<?= $investigation->title()->esc() ?>">
+        <?php snippet('picture', [
+            'file'       => $cover,
+            'alt'        => $investigation->title()->value(),
+            'sizes'      => '(min-width: 1000px) 465px, (min-width: 728px) 50vw, 100vw',
+            'srcsetName' => 'cover-card',
+            'lazy'       => true,
+        ]) ?>
     </figure>
     <?php endif ?>