test(e2e): deterministically wait for event stream in logout propagation test (#38535)

The test raced the server-side SSE channel registration: a logout
emitted before registration is silently dropped ([example
flake](https://github.com/go-gitea/gitea/actions/runs/29699349255/job/88225654080)).
The 500ms wait from https://github.com/go-gitea/gitea/pull/37403 only
made this unlikely.

The server now registers the channel before sending the initial response
bytes, so an open connection implies registration. The shared worker
forwards the built-in `open` event (replaying it to late-attaching ports
via `EventSource.readyState`), the page exposes it as a
`data-user-events-connected` attribute, and the test waits for that
attribute instead of a fixed timeout.

web_src/js/eventsource.sharedworker.ts

@@ -26,6 +26,11 @@ class Source {
       type: 'status',
       message: `registered to ${this.url}`,
     });
+
+    // replay the "open" event to ports attaching to an already-open source
+    if (this.eventSource?.readyState === EventSource.OPEN) {
+      port.postMessage({type: 'open'});
+    }
   }
 
   deregister(port: MessagePort) {

web_src/js/modules/worker.ts

@@ -54,6 +54,9 @@ export class UserEventsSharedWorker {
       } else if (event.data.type === 'close') {
         this.sharedWorker.port.postMessage({type: 'close'});
         this.sharedWorker.port.close();
+      } else if (event.data.type === 'open') {
+        // e2e tests wait for this attribute to know events cannot be missed anymore
+        document.documentElement.setAttribute('data-user-events-connected', 'true');
       }
       listener(event);
     });