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/webSession.test.ts

@@ -91,6 +91,44 @@ describe("WebSession unauthorized handling", () => {
   });
 });
 
+describe("WebSession default fetch receiver", () => {
+  /**
+   * Regression (live Firefox, ticket #13): the default global `fetch` was stored
+   * unbound, so `this.fetchImpl(…)` called it with the WebSession as receiver ⇒
+   * `TypeError: 'fetch' called on an object that does not implement interface
+   * Window` at pairing. jsdom does not enforce the receiver, so assert it here.
+   */
+  async function pairWithRecordingGlobalFetch(): Promise<unknown[]> {
+    const original = globalThis.fetch;
+    const receivers: unknown[] = [];
+    globalThis.fetch = function (this: unknown) {
+      receivers.push(this);
+      return Promise.resolve({
+        ok: true,
+        status: 200,
+        json: async () => ({ ok: true }),
+        text: async () => "{}",
+      });
+    } as unknown as typeof globalThis.fetch;
+    try {
+      // No `fetchImpl` injected ⇒ the global fallback is used.
+      const session = new WebSession({ baseUrl: "https://host", store: memStore() });
+      await session.pair("4821-93");
+    } finally {
+      globalThis.fetch = original;
+    }
+    return receivers;
+  }
+
+  it("pairs using the global fetch bound to globalThis, not the session", async () => {
+    const receivers = await pairWithRecordingGlobalFetch();
+
+    expect(receivers).toHaveLength(1);
+    expect(receivers[0]).toBe(globalThis);
+    expect(receivers[0]).not.toBeInstanceOf(WebSession);
+  });
+});
+
 describe("WebSession logout", () => {
   it("POSTs /api/logout same-origin and clears the flag on success", async () => {
     const store = memStore();