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/plugins/beforeafter/snippets/blocks/beforeafter.php

@@ -1,8 +1,13 @@
 <?php
 /** @var \Kirby\Cms\Block $block */
 $imageBefore = $block->imageBefore()->toFile();
-$imageAfter = $block->imageAfter()->toFile();
-$caption = $block->caption()->value();
+$imageAfter  = $block->imageAfter()->toFile();
+$caption     = $block->caption()->value();
+$imgSizes    = $imgSizes  ?? '(min-width: 768px) 50vw, 90vw';
+$imgSrcset   = $imgSrcset ?? 'body-half';
+
+$kirbyThumbs = kirby()->option('thumbs.srcsets', []);
+$srcsetAttr  = isset($kirbyThumbs[$imgSrcset]) ? fn($f) => $f->srcset($imgSrcset) : fn($f) => $f->srcset();
 ?>
 
 <?php if ($imageBefore || $imageAfter): ?>
@@ -11,12 +16,16 @@ $caption = $block->caption()->value();
         <?php if ($imageBefore): ?>
         <img class="image-before slider-image"
              src="<?= $imageBefore->url() ?>"
+             srcset="<?= $srcsetAttr($imageBefore) ?>"
+             sizes="<?= $imgSizes ?>"
              alt="<?= $imageBefore->alt()->or('Image avant')->esc() ?>" />
         <?php endif ?>
 
         <?php if ($imageAfter): ?>
         <img class="image-after slider-image"
              src="<?= $imageAfter->url() ?>"
+             srcset="<?= $srcsetAttr($imageAfter) ?>"
+             sizes="<?= $imgSizes ?>"
              alt="<?= $imageAfter->alt()->or('Image après')->esc() ?>" />
         <?php endif ?>
     </div>