fix(web): admin bibliothèques — correctifs complémentaires client et AdminPage

- web: AdminPage, client API (+ tests), styles

Refs: #15

apps/web/src/api/client.ts

@@ -56,15 +56,20 @@ function apiErrorMessage(detail: string, fallback: string): string {
   return fallback;
 }
 
+function requestHeaders(options: RequestOptions): Headers {
+  const headers = new Headers(options.headers);
+  if (options.body !== undefined && !headers.has("Content-Type")) {
+    headers.set("Content-Type", "application/json");
+  }
+  return headers;
+}
+
 async function request<T>(path: string, options: RequestOptions = {}): Promise<T> {
   try {
     const response = await fetch(`${API_BASE}${path}`, {
       ...options,
       credentials: "include",
-      headers: {
-        "Content-Type": "application/json",
-        ...options.headers
-      }
+      headers: requestHeaders(options)
     });
 
     if (!response.ok) {