Fix: treat an aborted SSE connect as a close, not a failure

Since the per-account push setup (#281), every account switch tears down
and re-creates push notifications for all connected clients. Aborting an
SSE connect that is still in flight lands in the fetch rejection handler,
which treated it as a network failure:

- fallbackToPolling() started an unsupervised 3s state poll on a client
  whose push had just been intentionally closed, after the cleanup that
  would have removed it had already run.
- The late rejection also nulled sseAbortController, orphaning the
  replacement connection set up right after: it could never be aborted
  again and reconnected itself in parallel once the server closed it.

Rapid switching multiplied both effects until the server's concurrency
limit stalled the app entirely.

Each connect attempt now tracks its own AbortController: an aborted
attempt returns silently instead of falling back to polling, and the
end-of-stream reconnect only fires if the stream is still the current
one. Regression tests simulate the switch churn both ways.

lib/__tests__/jmap-client-resilience.test.ts

@@ -349,6 +349,68 @@ describe('JMAPClient resilience', () => {
     });
   });
 
+  // Every account switch tears down and re-creates push for all connected
+  // clients. Aborting an SSE connect that is still in flight must read as an
+  // intentional close: treated as a network failure it spawns an unsupervised
+  // 3s polling interval per client, and its late rejection nulls the abort
+  // controller of the connection set up right after - which then can never be
+  // closed and reconnects itself in parallel. Rapid switching multiplies both
+  // until the server's concurrency limit stalls the app.
+  describe('SSE connect aborted mid-flight (account-switch churn)', () => {
+    function inFlightFetch(signals: AbortSignal[]) {
+      return (_url: RequestInfo | URL, init?: RequestInit) => {
+        if (init?.signal) signals.push(init.signal);
+        return new Promise<Response>((_resolve, reject) => {
+          const abort = () => reject(new DOMException('The operation was aborted.', 'AbortError'));
+          if (init?.signal?.aborted) return abort();
+          init?.signal?.addEventListener('abort', abort);
+        });
+      };
+    }
+
+    it('does not fall back to polling when the in-flight connect was aborted', async () => {
+      vi.useFakeTimers({ shouldAdvanceTime: false });
+      const client = await createConnectedClient();
+      fetchSpy.mockImplementation(inFlightFetch([]));
+
+      client.setupPushNotifications();
+      client.closePushNotifications();
+
+      // Flush the 1s network-error retry inside authenticatedFetch and a few
+      // would-be polling ticks (3s each).
+      await vi.advanceTimersByTimeAsync(10_000);
+
+      // The polling fallback is recognizable by its state-poll body; a
+      // keep-alive Core/echo that slips in must not fail the assertion.
+      const statePolls = fetchSpy.mock.calls.filter((call: unknown[]) => {
+        const body = (call[1] as RequestInit | undefined)?.body;
+        return typeof body === 'string' && body.includes('Mailbox/get');
+      });
+      expect(statePolls).toHaveLength(0);
+    });
+
+    it('keeps the replacement connection abortable when the aborted connect settles late', async () => {
+      vi.useFakeTimers({ shouldAdvanceTime: false });
+      const client = await createConnectedClient();
+      const signals: AbortSignal[] = [];
+      fetchSpy.mockImplementation(inFlightFetch(signals));
+
+      client.setupPushNotifications();
+      client.closePushNotifications();
+      client.setupPushNotifications();
+      // The retry inside authenticatedFetch re-sends the aborted first
+      // attempt later, so grab the replacement's signal now.
+      const replacementSignal = signals[signals.length - 1];
+
+      // Let the first attempt run through its retry and reject - after the
+      // replacement connect is already up.
+      await vi.advanceTimersByTimeAsync(2_000);
+
+      client.closePushNotifications();
+      expect(replacementSignal.aborted).toBe(true);
+    });
+  });
+
   describe('fetchBlobAsObjectUrl', () => {
     it('fetches blob with authentication and returns an object URL', async () => {
       const client = await createConnectedClient();