Backend: edition image d'entete + logo des adherents

- Champs upload image d'entete (coverUrl) et logo (logoUrl) dans le
  formulaire d'edition d'un adherent (composant ImageField, fichier ->
  data-URL via hidden input)
- updateMember persiste desormais coverUrl + logoUrl, et revalide la
  fiche publique /adherents/[id]
- serverActions.bodySizeLimit a 4mb (les data-URL depassent la limite
  par defaut de 1mb)

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

src/components/ImageField.tsx

@@ -0,0 +1,85 @@
+"use client";
+
+import { useRef, useState } from "react";
+
+export function ImageField({
+  name,
+  label,
+  defaultValue = "",
+  height = 130,
+}: {
+  name: string;
+  label: string;
+  defaultValue?: string;
+  height?: number;
+}) {
+  const [value, setValue] = useState(defaultValue);
+  const [error, setError] = useState<string | null>(null);
+  const fileRef = useRef<HTMLInputElement>(null);
+
+  function onFile(e: React.ChangeEvent<HTMLInputElement>) {
+    const f = e.target.files?.[0];
+    if (!f) return;
+    setError(null);
+    // Limite raisonnable : les data-URL transitent par une server action (~1 Mo max).
+    if (f.size > 900_000) {
+      setError("Image trop lourde (max ~900 Ko). Compressez-la d'abord.");
+      if (fileRef.current) fileRef.current.value = "";
+      return;
+    }
+    const reader = new FileReader();
+    reader.onload = () => setValue(String(reader.result));
+    reader.readAsDataURL(f);
+  }
+
+  function clear() {
+    setValue("");
+    setError(null);
+    if (fileRef.current) fileRef.current.value = "";
+  }
+
+  return (
+    <div>
+      <label className="field-label">{label}</label>
+      <input type="hidden" name={name} value={value} />
+
+      <div
+        style={{
+          height,
+          borderRadius: 12,
+          border: "1px dashed #d6c9ad",
+          background: value ? `center/cover no-repeat url(${value})` : "#f6efdc",
+          display: "flex",
+          alignItems: "center",
+          justifyContent: "center",
+          overflow: "hidden",
+          marginBottom: 10,
+        }}
+      >
+        {!value && <span style={{ fontSize: 13, color: "#a99c82" }}>Aucune image</span>}
+      </div>
+
+      <div style={{ display: "flex", gap: 8, alignItems: "center" }}>
+        <button
+          type="button"
+          onClick={() => fileRef.current?.click()}
+          style={{ border: "1px solid #d6c9ad", background: "#fff", color: "#6f6450", fontWeight: 600, fontSize: 13, padding: "8px 14px", borderRadius: 9, cursor: "pointer" }}
+        >
+          {value ? "Changer l'image" : "Choisir une image"}
+        </button>
+        {value && (
+          <button
+            type="button"
+            onClick={clear}
+            style={{ border: "none", background: "none", color: "#d8472b", fontWeight: 600, fontSize: 13, cursor: "pointer" }}
+          >
+            Retirer
+          </button>
+        )}
+        <input ref={fileRef} type="file" accept="image/*" onChange={onFile} style={{ display: "none" }} />
+      </div>
+
+      {error && <div style={{ fontSize: 12.5, color: "#a3372e", marginTop: 6 }}>{error}</div>}
+    </div>
+  );
+}