feat: adaptive promo images with blurred fill

Replace cover-crop background with PromoImage component: sharp
contain image over a blurred cover copy. Whole image visible on
any aspect ratio, no flat letterbox bars. Applied to accueil,
adherent profile, backend promotions, member space preview +
upload dropzone.

Also retheme adherent logo frame (cream gradient + warm border)
instead of flat white.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/PromoImage.tsx

@@ -0,0 +1,39 @@
+/**
+ * Promo image that adapts to any aspect ratio.
+ * Sharp `contain` image on top of a blurred `cover` copy of itself,
+ * so the whole image is always visible with no flat letterbox bars.
+ * Render inside a `position: relative` box; place after it any
+ * badges/labels that must sit on top.
+ */
+export function PromoImage({ src, alt = "" }: { src: string; alt?: string }) {
+  return (
+    <>
+      <div
+        aria-hidden
+        style={{
+          position: "absolute",
+          inset: 0,
+          backgroundImage: `url(${src})`,
+          backgroundSize: "cover",
+          backgroundPosition: "center",
+          filter: "blur(18px) saturate(1.15)",
+          transform: "scale(1.18)",
+        }}
+      />
+      <div aria-hidden style={{ position: "absolute", inset: 0, background: "rgba(40,30,15,0.16)" }} />
+      {/* eslint-disable-next-line @next/next/no-img-element */}
+      <img
+        src={src}
+        alt={alt}
+        style={{
+          position: "absolute",
+          inset: 0,
+          width: "100%",
+          height: "100%",
+          objectFit: "contain",
+          display: "block",
+        }}
+      />
+    </>
+  );
+}