perf: replace real-time polling with SSE streaming

Replace 5-second setInterval polling with EventSource connection to the
new /realtime/stream SSE endpoint. The server pushes visitor updates
instead of each client independently polling. Auto-reconnects on
connection drops.

CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 ### Improved
 
 - **Even faster dashboard loading.** Your dashboard now fetches all its data — pages, locations, devices, referrers, performance, and goals — in a single request instead of seven separate ones. This means the entire dashboard appears at once rather than sections loading one by one, and puts much less strain on the server when many people are viewing their analytics at the same time.
+- **Smoother real-time updates.** The real-time visitors page now streams updates instantly from the server instead of checking for new data every few seconds. Visitors appear and disappear in real-time with no delay, and the page uses far fewer server resources — especially when many people are watching their live traffic at the same time.
 
 ### Added