Feat: images responsives vignettes Play + composant ResponsivePicture

- Config: presets thumbnail + thumbnail-webp (170/255/355/510/710w)
- play.json.php: expose thumbnailSrcset + thumbnailWebp
- Nouveau composant ResponsivePicture.svelte (src, srcset, webp, sizes, alt, cls)
- Play.svelte: utilise ResponsivePicture dans le carousel
  sizes="clamp(170px, 18.41vw, 355px)"

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

src/components/ui/ResponsivePicture.svelte

@@ -0,0 +1,28 @@
+<script>
+  /**
+   * Image responsive avec support WebP via <picture>.
+   *
+   * @prop {string}  src    — URL fallback (format original)
+   * @prop {string}  srcset — srcset format original (ex: "200w ..., 400w ...")
+   * @prop {string}  webp   — srcset format WebP
+   * @prop {string}  sizes  — valeur de l'attribut sizes
+   * @prop {string}  [alt]  — texte alternatif
+   * @prop {string}  [cls]  — classe CSS à appliquer sur <img>
+   */
+  let { src, srcset, webp, sizes, alt = '', cls = '' } = $props()
+</script>
+
+<picture>
+  {#if webp}
+    <source type="image/webp" srcset={webp} {sizes} />
+  {/if}
+  <img
+    {src}
+    {srcset}
+    {sizes}
+    {alt}
+    class={cls}
+    loading="lazy"
+    decoding="async"
+  />
+</picture>