feat: responsive images for investigation template

Add srcsets in config:
- cover-hero: 940px fixe ≥1000px, 90vw en dessous
- body-full: 100vw (1 colonne)
- body-half: 50vw ≥768px, 90vw en dessous (2 colonnes)

Apply in templates/snippets:
- Cover hero → snippet picture + cover-hero
- Body 1-col images → snippet picture + body-full
- Body 2-col images → snippet picture + body-half
- gallery.php → snippet picture (sizes/srcset paramétrables)
- beforeafter.php → srcset/sizes sur <img> (pas de <picture> pour ne pas
  casser le JS du slider)

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

site/snippets/blocks/gallery.php

@@ -1,6 +1,8 @@
 <?php
 /** @var \Kirby\Cms\Block $block */
-$images = $block->images()->toFiles();
+$images      = $block->images()->toFiles();
+$imgSizes    = $imgSizes    ?? '(min-width: 768px) 50vw, 90vw';
+$imgSrcset   = $imgSrcset   ?? 'body-half';
 ?>
 
 <div class="swiper">
@@ -8,7 +10,13 @@ $images = $block->images()->toFiles();
         <?php foreach ($images as $image): ?>
         <div class="swiper-slide">
             <figure>
-                <img src="<?= $image->url() ?>" alt="<?= $image->alt()->esc() ?>">
+                <?php snippet('picture', [
+                    'file'       => $image,
+                    'alt'        => $image->alt()->value(),
+                    'sizes'      => $imgSizes,
+                    'srcsetName' => $imgSrcset,
+                    'lazy'       => true,
+                ]) ?>
                 <?php if ($image->caption()->isNotEmpty()): ?>
                 <figcaption><?= $image->caption()->html() ?></figcaption>
                 <?php endif ?>