feat(realtime): long polling for app-wide events behind proxies

When a proxy lets neither WebSocket nor SSE through, the app opens a
poll connection: the server buffers its events (an SSEClient without a
stream), each GET waits up to 25 s, and room joins work like for SSE.
Tasks and lists now update live there too, and note sync takes its
notifications from it instead of a second long poll. Idle connections
close after two minutes. The profile shows such a connection as limited.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/__tests__/useConnectionState.test.tsx

@@ -1,10 +1,10 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { act, renderHook } from '@testing-library/react';
 
-let statusListener: ((connected: boolean) => void) | null = null;
+let statusListener: ((connected: boolean, transport?: string) => void) | null = null;
 let currentConnected = false;
 vi.mock('../api', () => ({
-  onRealtimeStatus: (cb: (connected: boolean) => void) => {
+  onRealtimeStatus: (cb: (connected: boolean, transport?: string) => void) => {
     statusListener = cb;
     cb(currentConnected);
     return () => (statusListener = null);
@@ -86,6 +86,15 @@ describe('useConnectionState', () => {
     expect(fetchMock.mock.calls.length).toBe(calls);
   });
 
+  it('is limited while changes only come by long polling', async () => {
+    const { result } = renderHook(() => useConnectionState());
+    act(() => statusListener?.(true, 'poll'));
+    expect(result.current).toBe('limited');
+    expect(fetchMock).not.toHaveBeenCalled();
+    act(() => statusListener?.(true, 'sse'));
+    expect(result.current).toBe('online');
+  });
+
   it('repeated failed reconnects do not postpone the check', async () => {
     currentConnected = true;
     const { result } = renderHook(() => useConnectionState());