Fix: prevent mobile web remote desktop freeze on Safari app-switch

by forcing clean WS reconnect on foreground return;

add 30-second grace period on server to avoid cold-start on quick reconnects

server/web/index.html

@@ -4185,11 +4185,33 @@
                     clearTimeout(backgroundDisconnectTimer);
                     backgroundDisconnectTimer = null;
                 }
-                // Reconnect or send immediate ping
+                const screenPage = document.getElementById('screen-page');
+                const onScreenPage = screenPage && screenPage.classList.contains('active') && currentDevice;
+
                 if (!ws || ws.readyState !== WebSocket.OPEN) {
+                    // WS was dropped — reset decoder before reconnect.
+                    if (onScreenPage && decoder) {
+                        try { decoder.close(); } catch(e) {}
+                        decoder = null;
+                        needKeyframe = true;
+                    }
+                    connectWebSocket();
+                } else if (onScreenPage && isTouchDevice) {
+                    // iOS suspends the tab and silently invalidates the VideoDecoder
+                    // GPU context (or quietly closes the WS while readyState stays OPEN).
+                    // Always force a clean WS reconnect on mobile so the server delivers
+                    // a fresh stream start; the server no longer sends a stale cached
+                    // keyframe (cleared on disconnect), so the client waits for the next
+                    // natural IDR — no mosaic, no freeze.
+                    if (decoder) {
+                        try { decoder.close(); } catch(e) {}
+                        decoder = null;
+                        needKeyframe = true;
+                    }
+                    ws.onclose = null;
+                    ws.close();
                     connectWebSocket();
                 } else if (token) {
-                    // Connection still open - send immediate ping to refresh server heartbeat
                     ws.send(JSON.stringify({ cmd: 'ping', token }));
                 }
             }