Feat: images responsives mockup Portfolio via ResponsivePicture

- Config: presets mockup + mockup-webp (350/480/700/960w)
- portfolio.json.php: expose mockupSrcset + mockupWebp
- Portfolio.svelte: remplace <img> par ResponsivePicture
  sizes="(max-width: 700px) 90vw, 25vw"

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

site/config/config.php

@@ -108,6 +108,19 @@ return [
                 '600w' => ['width' => 600, 'format' => 'webp'],
                 '800w' => ['width' => 800, 'format' => 'webp'],
             ],
+            // Mockup portfolio — desktop ~25vw, mobile ~90vw
+            'mockup' => [
+                '350w' => ['width' => 350],
+                '480w' => ['width' => 480],
+                '700w' => ['width' => 700],
+                '960w' => ['width' => 960],
+            ],
+            'mockup-webp' => [
+                '350w' => ['width' => 350, 'format' => 'webp'],
+                '480w' => ['width' => 480, 'format' => 'webp'],
+                '700w' => ['width' => 700, 'format' => 'webp'],
+                '960w' => ['width' => 960, 'format' => 'webp'],
+            ],
             // Vignettes jeux — active: clamp(170px, 18.41vw, 355px), retina 2x → 710px
             'thumbnail' => [
                 '170w' => ['width' => 170],