feat(auth): use the cookie session and refresh on 401

The token is no longer stored; a tbp_session flag remembers the
login. apiFetch renews the session once on a 401 and sends the
request again. Parallel requests and other tabs share one refresh,
because Auth revokes the session when a refresh token is used twice.
Only a 401 from the refresh ends the session; a 502 or no network
shows the normal error.

src/components/BacklogCard.test.jsx

@@ -12,7 +12,7 @@ const saveButton = () => screen.getByRole('button', {name: 'BACKLOG.SAVE'});
 
 describe('BacklogCard', () => {
     beforeEach(() => {
-        localStorage.setItem('tbp_jwt', 'token');
+        localStorage.setItem('tbp_session', '1');
         globalThis.fetch = vi.fn(async (url, init) => ({ok: true, status: 200, json: async () => JSON.parse(init.body)}));
     });