feat(ui): implement new event stream connection logic

Added a new event stream connection method to enhance the handling of
server events. This includes a reconnect mechanism for improved reliability
in case of connection errors. The configuration now allows toggling the
new event stream feature via `devNewEventStream`. Additionally, tests
were added to ensure the new functionality works as expected, including
reconnection behavior after an error.

Signed-off-by: Deluan <deluan@navidrome.org>

ui/src/eventStream.test.js

@@ -0,0 +1,49 @@
+import { describe, it, beforeEach, vi, expect } from 'vitest'
+import { startEventStream } from './eventStream'
+import { serverDown } from './actions'
+import config from './config'
+
+class MockEventSource {
+  constructor(url) {
+    this.url = url
+    this.readyState = 1
+    this.listeners = {}
+    this.onerror = null
+  }
+  addEventListener(type, handler) {
+    this.listeners[type] = handler
+  }
+  close() {
+    this.readyState = 2
+  }
+}
+
+describe('startEventStream', () => {
+  vi.useFakeTimers()
+  let dispatch
+  let instance
+
+  beforeEach(() => {
+    dispatch = vi.fn()
+    global.EventSource = vi.fn((url) => {
+      instance = new MockEventSource(url)
+      return instance
+    })
+    localStorage.setItem('is-authenticated', 'true')
+    localStorage.setItem('token', 'abc')
+    config.devNewEventStream = true
+  })
+
+  afterEach(() => {
+    config.devNewEventStream = false
+  })
+
+  it('reconnects after an error', async () => {
+    await startEventStream(dispatch)
+    expect(global.EventSource).toHaveBeenCalledTimes(1)
+    instance.onerror(new Event('error'))
+    expect(dispatch).toHaveBeenCalledWith(serverDown())
+    vi.advanceTimersByTime(5000)
+    expect(global.EventSource).toHaveBeenCalledTimes(2)
+  })
+})