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.

routers/web/events/events.go

@@ -38,7 +38,6 @@ func Events(ctx *context.Context) {
 
 	// Listen to connection close and un-register messageChan
 	notify := ctx.Done()
-	ctx.Resp.Flush()
 
 	shutdownCtx := graceful.GetManager().ShutdownContext()
 
@@ -57,11 +56,14 @@ func Events(ctx *context.Context) {
 		}
 	}
 
+	// send the initial response bytes only after registering messageChan, so a client whose
+	// connection is open can rely on receiving all subsequent events
 	if _, err := ctx.Resp.Write([]byte("\n")); err != nil {
 		log.Error("Unable to write to EventStream: %v", err)
 		unregister()
 		return
 	}
+	ctx.Resp.Flush()
 
 	timer := time.NewTicker(30 * time.Second)