fix(frontend): binder le fetch global à globalThis (pairing Firefox) (#13)

Firefox exige que fetch soit appelé sur Window : passer la référence nue
`fetch` en dépendance déclenchait « 'fetch' called on an object that does
not implement interface Window » au pairing. Nouveau helper defaultFetch()
qui binde globalThis.fetch, utilisé par webSession.

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

frontend/src/adapters/http/httpInvoker.ts

@@ -41,6 +41,29 @@ export type FetchLike = (
   text(): Promise<string>;
 }>;
 
+/**
+ * Resolves the default `fetch`, **bound to `globalThis`**.
+ *
+ * WHATWG `fetch` is a method of the global object and checks its receiver: once
+ * stored in a field and called as `this.fetchImpl(…)`, its `this` is the owning
+ * adapter instance, not `window`. A real browser rejects that — Firefox with
+ * `TypeError: 'fetch' called on an object that does not implement interface
+ * Window`, Chrome with "Illegal invocation". jsdom does not enforce the receiver,
+ * so the unit tests never caught it; the bind is what keeps the fallback callable
+ * from a field. An *injected* `fetchImpl` is left untouched (a test stub is a
+ * plain function and needs no receiver).
+ *
+ * When there is no global `fetch` (SSR / bare Node), the value is returned as-is
+ * so the failure still happens at call time, exactly as before, rather than
+ * throwing during construction.
+ */
+export function defaultFetch(): FetchLike {
+  const globalFetch = globalThis.fetch;
+  return (
+    typeof globalFetch === "function" ? globalFetch.bind(globalThis) : globalFetch
+  ) as unknown as FetchLike;
+}
+
 /** Configuration for the HTTP invoker. */
 export interface HttpInvokerConfig {
   /** Absolute base URL of the backend, e.g. `https://host:port`. No trailing slash. */
@@ -85,10 +108,8 @@ export class HttpInvoker {
     this.baseUrl = config.baseUrl.replace(/\/+$/, "");
     this.token = config.token;
     this.onUnauthorized = config.onUnauthorized;
-    // `globalThis.fetch` exists in the browser (and jsdom); the cast narrows it
-    // to the minimal shape used here.
-    this.fetchImpl =
-      config.fetchImpl ?? (globalThis.fetch as unknown as FetchLike);
+    // The global fallback must stay bound to `globalThis` — see `defaultFetch`.
+    this.fetchImpl = config.fetchImpl ?? defaultFetch();
   }
 
   /**