fix: refresh all dashboard thumbnails on tab return and add periodic fallback

Two issues caused stale thumbnails on the dashboard:

1. Returning to the dashboard tab only processed the (empty) refresh
   queue — it never triggered a full refresh. Now, if the dashboard was
   hidden for more than 3 seconds, all ETags are cleared and refreshAll()
   is called so every tile fetches a fresh screenshot.

2. If SSE activity events were missed (e.g. during reconnect), tiles
   would never update until the next terminal activity. Added a 10s
   periodic fallback that calls refreshAll() when the dashboard is
   visible, ensuring tiles stay reasonably current even without SSE.

webterm/server.go

@@ -1486,16 +1486,38 @@ func (s *LocalServer) handleRoot(w http.ResponseWriter, r *http.Request) {
 			return document.visibilityState === 'visible';
 		}
 
+		let dashboardHiddenAt = 0;
+
 		function onDashboardFocusChanged() {
 			if (dashboardCanRequestScreenshots()) {
+				// If the dashboard was hidden for more than a few seconds,
+				// clear ETags and refresh all tiles so the user sees current state.
+				const away = dashboardHiddenAt ? (Date.now() - dashboardHiddenAt) : 0;
+				dashboardHiddenAt = 0;
+				if (away > 3000) {
+					for (const key in etagBySlug) {
+						delete etagBySlug[key];
+					}
+					refreshAll();
+				} else {
 					processRefreshQueue();
 				}
+			} else {
+				dashboardHiddenAt = Date.now();
+			}
 		}
 
 		document.addEventListener('visibilitychange', onDashboardFocusChanged);
 		window.addEventListener('focus', onDashboardFocusChanged);
 		window.addEventListener('blur', onDashboardFocusChanged);
 
+		// Periodic fallback: refresh all tiles every 10s in case SSE events were missed.
+		setInterval(() => {
+			if (dashboardCanRequestScreenshots()) {
+				refreshAll();
+			}
+		}, 10000);
+
 		function processRefreshQueue() {
 			if (refreshQueue.length === 0 || !dashboardCanRequestScreenshots()) return;